fix(otel): make HTTP trace export survive app-server runtimes (#14300)

## Summary

This PR fixes OTLP HTTP trace export in runtimes where the previous
exporter setup was unreliable, especially around app-server usage. It
also removes the old `codex_otel::otel_provider` compatibility shim and
switches remaining call sites over to the crate-root
`codex_otel::OtelProvider` export.

## What changed

- Use a runtime-safe OTLP HTTP trace exporter path for Tokio runtimes.
- Add an async HTTP client path for trace export when we are already
inside a multi-thread Tokio runtime.
- Make provider shutdown flush traces before tearing down the tracer
provider.
- Add loopback coverage that verifies traces are actually sent to
`/v1/traces`:
  - outside Tokio
  - inside a multi-thread Tokio runtime
  - inside a current-thread Tokio runtime
- Remove the `codex_otel::otel_provider` shim and update remaining
imports.

## Why

I hit cases where spans were being created correctly but never made it
to the collector. The issue turned out to be in exporter/runtime
behavior rather than the span plumbing itself. This PR narrows that gap
and gives us regression coverage for the actual export path.

codex-rs/otel/tests/suite/otlp_http_loopback.rs

@@ -1,5 +1,7 @@
+use codex_otel::OtelProvider;
 use codex_otel::config::OtelExporter;
 use codex_otel::config::OtelHttpProtocol;
+use codex_otel::config::OtelSettings;
 use codex_otel::metrics::MetricsClient;
 use codex_otel::metrics::MetricsConfig;
 use codex_otel::metrics::Result;
@@ -8,10 +10,12 @@ use std::io::Read as _;
 use std::io::Write as _;
 use std::net::TcpListener;
 use std::net::TcpStream;
+use std::path::PathBuf;
 use std::sync::mpsc;
 use std::thread;
 use std::time::Duration;
 use std::time::Instant;
+use tracing_subscriber::layer::SubscriberExt;
 
 struct CapturedRequest {
     path: String,
@@ -212,3 +216,346 @@ fn otlp_http_exporter_sends_metrics_to_collector() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn otlp_http_exporter_sends_traces_to_collector()
+-> std::result::Result<(), Box<dyn std::error::Error>> {
+    let listener = TcpListener::bind("127.0.0.1:0").expect("bind");
+    let addr = listener.local_addr().expect("local_addr");
+    listener.set_nonblocking(true).expect("set_nonblocking");
+
+    let (tx, rx) = mpsc::channel::<Vec<CapturedRequest>>();
+    let server = thread::spawn(move || {
+        let mut captured = Vec::new();
+        let deadline = Instant::now() + Duration::from_secs(3);
+
+        while Instant::now() < deadline {
+            match listener.accept() {
+                Ok((mut stream, _)) => {
+                    let result = read_http_request(&mut stream);
+                    let _ = write_http_response(&mut stream, "202 Accepted");
+                    if let Ok((path, headers, body)) = result {
+                        captured.push(CapturedRequest {
+                            path,
+                            content_type: headers.get("content-type").cloned(),
+                            body,
+                        });
+                    }
+                }
+                Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {
+                    thread::sleep(Duration::from_millis(10));
+                }
+                Err(_) => break,
+            }
+        }
+
+        let _ = tx.send(captured);
+    });
+
+    let otel = OtelProvider::from(&OtelSettings {
+        environment: "test".to_string(),
+        service_name: "codex-cli".to_string(),
+        service_version: env!("CARGO_PKG_VERSION").to_string(),
+        codex_home: PathBuf::from("."),
+        exporter: OtelExporter::None,
+        trace_exporter: OtelExporter::OtlpHttp {
+            endpoint: format!("http://{addr}/v1/traces"),
+            headers: HashMap::new(),
+            protocol: OtelHttpProtocol::Json,
+            tls: None,
+        },
+        metrics_exporter: OtelExporter::None,
+        runtime_metrics: false,
+    })?
+    .expect("otel provider");
+    let tracing_layer = otel.tracing_layer().expect("tracing layer");
+    let subscriber = tracing_subscriber::registry().with(tracing_layer);
+
+    tracing::subscriber::with_default(subscriber, || {
+        let span = tracing::info_span!(
+            "trace-loopback",
+            otel.name = "trace-loopback",
+            otel.kind = "server",
+            rpc.system = "jsonrpc",
+            rpc.method = "trace-loopback",
+        );
+        let _guard = span.enter();
+        tracing::info!("trace loopback event");
+    });
+    otel.shutdown();
+
+    server.join().expect("server join");
+    let captured = rx.recv_timeout(Duration::from_secs(1)).expect("captured");
+
+    let request = captured
+        .iter()
+        .find(|req| req.path == "/v1/traces")
+        .unwrap_or_else(|| {
+            let paths = captured
+                .iter()
+                .map(|req| req.path.as_str())
+                .collect::<Vec<_>>()
+                .join(", ");
+            panic!(
+                "missing /v1/traces request; got {}: {paths}",
+                captured.len()
+            );
+        });
+    let content_type = request
+        .content_type
+        .as_deref()
+        .unwrap_or("<missing content-type>");
+    assert!(
+        content_type.starts_with("application/json"),
+        "unexpected content-type: {content_type}"
+    );
+
+    let body = String::from_utf8_lossy(&request.body);
+    assert!(
+        body.contains("trace-loopback"),
+        "expected span name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+    assert!(
+        body.contains("codex-cli"),
+        "expected service name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn otlp_http_exporter_sends_traces_to_collector_in_tokio_runtime()
+-> std::result::Result<(), Box<dyn std::error::Error>> {
+    let listener = TcpListener::bind("127.0.0.1:0").expect("bind");
+    let addr = listener.local_addr().expect("local_addr");
+    listener.set_nonblocking(true).expect("set_nonblocking");
+
+    let (tx, rx) = mpsc::channel::<Vec<CapturedRequest>>();
+    let server = thread::spawn(move || {
+        let mut captured = Vec::new();
+        let deadline = Instant::now() + Duration::from_secs(3);
+
+        while Instant::now() < deadline {
+            match listener.accept() {
+                Ok((mut stream, _)) => {
+                    let result = read_http_request(&mut stream);
+                    let _ = write_http_response(&mut stream, "202 Accepted");
+                    if let Ok((path, headers, body)) = result {
+                        captured.push(CapturedRequest {
+                            path,
+                            content_type: headers.get("content-type").cloned(),
+                            body,
+                        });
+                    }
+                }
+                Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {
+                    thread::sleep(Duration::from_millis(10));
+                }
+                Err(_) => break,
+            }
+        }
+
+        let _ = tx.send(captured);
+    });
+
+    let otel = OtelProvider::from(&OtelSettings {
+        environment: "test".to_string(),
+        service_name: "codex-cli".to_string(),
+        service_version: env!("CARGO_PKG_VERSION").to_string(),
+        codex_home: PathBuf::from("."),
+        exporter: OtelExporter::None,
+        trace_exporter: OtelExporter::OtlpHttp {
+            endpoint: format!("http://{addr}/v1/traces"),
+            headers: HashMap::new(),
+            protocol: OtelHttpProtocol::Json,
+            tls: None,
+        },
+        metrics_exporter: OtelExporter::None,
+        runtime_metrics: false,
+    })?
+    .expect("otel provider");
+    let tracing_layer = otel.tracing_layer().expect("tracing layer");
+    let subscriber = tracing_subscriber::registry().with(tracing_layer);
+
+    tracing::subscriber::with_default(subscriber, || {
+        let span = tracing::info_span!(
+            "trace-loopback-tokio",
+            otel.name = "trace-loopback-tokio",
+            otel.kind = "server",
+            rpc.system = "jsonrpc",
+            rpc.method = "trace-loopback-tokio",
+        );
+        let _guard = span.enter();
+        tracing::info!("trace loopback event from tokio runtime");
+    });
+    otel.shutdown();
+
+    server.join().expect("server join");
+    let captured = rx.recv_timeout(Duration::from_secs(1)).expect("captured");
+
+    let request = captured
+        .iter()
+        .find(|req| req.path == "/v1/traces")
+        .unwrap_or_else(|| {
+            let paths = captured
+                .iter()
+                .map(|req| req.path.as_str())
+                .collect::<Vec<_>>()
+                .join(", ");
+            panic!(
+                "missing /v1/traces request; got {}: {paths}",
+                captured.len()
+            );
+        });
+    let content_type = request
+        .content_type
+        .as_deref()
+        .unwrap_or("<missing content-type>");
+    assert!(
+        content_type.starts_with("application/json"),
+        "unexpected content-type: {content_type}"
+    );
+
+    let body = String::from_utf8_lossy(&request.body);
+    assert!(
+        body.contains("trace-loopback-tokio"),
+        "expected span name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+    assert!(
+        body.contains("codex-cli"),
+        "expected service name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+
+    Ok(())
+}
+
+#[test]
+fn otlp_http_exporter_sends_traces_to_collector_in_current_thread_tokio_runtime()
+-> std::result::Result<(), Box<dyn std::error::Error>> {
+    let listener = TcpListener::bind("127.0.0.1:0").expect("bind");
+    let addr = listener.local_addr().expect("local_addr");
+    listener.set_nonblocking(true).expect("set_nonblocking");
+
+    let (tx, rx) = mpsc::channel::<Vec<CapturedRequest>>();
+    let server = thread::spawn(move || {
+        let mut captured = Vec::new();
+        let deadline = Instant::now() + Duration::from_secs(3);
+
+        while Instant::now() < deadline {
+            match listener.accept() {
+                Ok((mut stream, _)) => {
+                    let result = read_http_request(&mut stream);
+                    let _ = write_http_response(&mut stream, "202 Accepted");
+                    if let Ok((path, headers, body)) = result {
+                        captured.push(CapturedRequest {
+                            path,
+                            content_type: headers.get("content-type").cloned(),
+                            body,
+                        });
+                    }
+                }
+                Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {
+                    thread::sleep(Duration::from_millis(10));
+                }
+                Err(_) => break,
+            }
+        }
+
+        let _ = tx.send(captured);
+    });
+
+    let (runtime_result_tx, runtime_result_rx) = mpsc::channel::<std::result::Result<(), String>>();
+    let runtime_thread = thread::spawn(move || {
+        let runtime = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .expect("current-thread runtime");
+
+        let result = runtime.block_on(async move {
+            let otel = OtelProvider::from(&OtelSettings {
+                environment: "test".to_string(),
+                service_name: "codex-cli".to_string(),
+                service_version: env!("CARGO_PKG_VERSION").to_string(),
+                codex_home: PathBuf::from("."),
+                exporter: OtelExporter::None,
+                trace_exporter: OtelExporter::OtlpHttp {
+                    endpoint: format!("http://{addr}/v1/traces"),
+                    headers: HashMap::new(),
+                    protocol: OtelHttpProtocol::Json,
+                    tls: None,
+                },
+                metrics_exporter: OtelExporter::None,
+                runtime_metrics: false,
+            })
+            .map_err(|err| err.to_string())?
+            .expect("otel provider");
+            let tracing_layer = otel.tracing_layer().expect("tracing layer");
+            let subscriber = tracing_subscriber::registry().with(tracing_layer);
+
+            tracing::subscriber::with_default(subscriber, || {
+                let span = tracing::info_span!(
+                    "trace-loopback-current-thread",
+                    otel.name = "trace-loopback-current-thread",
+                    otel.kind = "server",
+                    rpc.system = "jsonrpc",
+                    rpc.method = "trace-loopback-current-thread",
+                );
+                let _guard = span.enter();
+                tracing::info!("trace loopback event from current-thread tokio runtime");
+            });
+            otel.shutdown();
+            Ok::<(), String>(())
+        });
+        let _ = runtime_result_tx.send(result);
+    });
+
+    runtime_result_rx
+        .recv_timeout(Duration::from_secs(5))
+        .expect("current-thread runtime should complete")
+        .map_err(std::io::Error::other)?;
+    runtime_thread.join().expect("runtime thread");
+
+    server.join().expect("server join");
+    let captured = rx.recv_timeout(Duration::from_secs(1)).expect("captured");
+
+    let request = captured
+        .iter()
+        .find(|req| req.path == "/v1/traces")
+        .unwrap_or_else(|| {
+            let paths = captured
+                .iter()
+                .map(|req| req.path.as_str())
+                .collect::<Vec<_>>()
+                .join(", ");
+            panic!(
+                "missing /v1/traces request; got {}: {paths}",
+                captured.len()
+            );
+        });
+    let content_type = request
+        .content_type
+        .as_deref()
+        .unwrap_or("<missing content-type>");
+    assert!(
+        content_type.starts_with("application/json"),
+        "unexpected content-type: {content_type}"
+    );
+
+    let body = String::from_utf8_lossy(&request.body);
+    assert!(
+        body.contains("trace-loopback-current-thread"),
+        "expected span name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+    assert!(
+        body.contains("codex-cli"),
+        "expected service name not found; body prefix: {}",
+        &body.chars().take(2000).collect::<String>()
+    );
+
+    Ok(())
+}