feat(app-server-test-client): support tracing

[owenlin0]

Summary

This adds tracing support to the app-server-test-client so requests it sends to app-server carry W3C trace context, and each run prints an easy-to-follow trace link at the end. Depends on #13285

Example:

$ cargo run -p codex-app-server-test-client -- \
  --codex-bin target/debug/codex \
  send-message-v2 "hello"
  
// truncated for brevity...
< {
<   "method": "turn/completed",
<   "params": {
<     "threadId": "019cb0f6-412e-7b43-a031-72461a990bea",
<     "turn": {
<       "error": null,
<       "id": "019cb0f6-4151-7ab0-af08-7f18a16612d6",
<       "items": [],
<       "status": "completed"
<     }
<   }
< }

< turn/completed notification: Completed

[Datadog trace]
go/trace/eaa4e7d57f804ca89017e62ce9f88de8

[codex app-server exited: exit status: 0]

What changed

• Attach traceparent metadata to outgoing JSON-RPC requests from the test client
• Generate a trace ID and root span ID per client session
• Print a Datadog trace summary at the end of each run so it’s easier to jump from a local repro to tracing
• Wrap the main command flows so the trace summary still gets printed after the request lifecycle completes
• Improve spawned app-server shutdown behavior by waiting briefly for a clean exit before killing the process (otherwise the trace gets dropped)

Why

The test client is useful for reproducing app-server behavior locally, but it was hard to connect a local run to backend traces. With this change, local debugging gets a lot easier: you can run the client, reproduce the issue, and immediately inspect the corresponding trace.

[owenlin0]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

codex/codex-rs/app-server-test-client/src/lib.rs

Lines 703 to 715 in 4d7d376

	let mut client = CodexClient::connect(endpoint, config_overrides)?;
	let initialize = client.initialize()?;
	println!("< initialize response: {initialize:?}");
	let resume_response = client.thread_resume(ThreadResumeParams {
	thread_id,
	..Default::default()
	})?;
	println!("< thread/resume response: {resume_response:?}");
	println!("< streaming notifications until process is terminated");
	client.stream_notifications_forever()

Route streaming commands through trace-summary wrapper

The commit message says each run should print a Datadog trace link, but thread_resume_follow (and similarly watch) still instantiate CodexClient directly and return stream_notifications_forever() without calling print_trace_summary(). If these commands end due disconnect/error, no trace URL is emitted.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".