feat(core, tracing): create turn spans over websockets (#14632)

## Description

Dependent on:
- [responsesapi] https://github.com/openai/openai/pull/760991 
- [codex-backend] https://github.com/openai/openai/pull/760985

`codex app-server -> codex-backend -> responsesapi` now reuses a
persistent websocket connection across many turns. This PR updates
tracing when using websockets so that each `response.create` websocket
request propagates the current tracing context, so we can get a holistic
end-to-end trace for each turn.

Tracing is propagated via special keys (`ws_request_header_traceparent`,
`ws_request_header_tracestate`) set in the `client_metadata` param in
Responses API.

Currently tracing on websockets is a bit broken because we only set
tracing context on ws connection time, so it's detached from a
`turn/start` request.

codex-rs/core/tests/common/tracing.rs

@@ -0,0 +1,26 @@
+use opentelemetry::global;
+use opentelemetry::trace::TracerProvider as _;
+use opentelemetry_sdk::propagation::TraceContextPropagator;
+use opentelemetry_sdk::trace::SdkTracerProvider;
+use tracing::dispatcher::DefaultGuard;
+use tracing_subscriber::layer::SubscriberExt;
+use tracing_subscriber::util::SubscriberInitExt;
+
+pub struct TestTracingContext {
+    _provider: SdkTracerProvider,
+    _guard: DefaultGuard,
+}
+
+pub fn install_test_tracing(tracer_name: &str) -> TestTracingContext {
+    global::set_text_map_propagator(TraceContextPropagator::new());
+
+    let provider = SdkTracerProvider::builder().build();
+    let tracer = provider.tracer(tracer_name.to_string());
+    let subscriber =
+        tracing_subscriber::registry().with(tracing_opentelemetry::layer().with_tracer(tracer));
+
+    TestTracingContext {
+        _provider: provider,
+        _guard: subscriber.set_default(),
+    }
+}