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/src/trace_context.rs

@@ -17,7 +17,11 @@ const TRACESTATE_ENV_VAR: &str = "TRACESTATE";
 static TRACEPARENT_CONTEXT: OnceLock<Option<Context>> = OnceLock::new();
 
 pub fn current_span_w3c_trace_context() -> Option<W3cTraceContext> {
-    let context = Span::current().context();
+    span_w3c_trace_context(&Span::current())
+}
+
+pub fn span_w3c_trace_context(span: &Span) -> Option<W3cTraceContext> {
+    let context = span.context();
     if !context.span().span_context().is_valid() {
         return None;
     }