[rollout-trace] Include x-request-id in rollout trace.

[cassirer-openai]

Why

Rollout traces need an identifier that can be used to correlate a Codex inference with upstream Responses API, proxy, and engine logs. The reduced trace model already exposed upstream_request_id, but it was being populated from the Responses API response.id. That value is useful for previous_response_id chaining, but it is not the transport request id that upstream systems key on.

This PR separates those concepts so trace consumers can reliably answer both questions:

• which Responses API response did this inference produce?
• which upstream request handled it?

Structure

The change keeps the upstream request id at the same lifecycle level as the provider stream:

• codex-api captures the x-request-id HTTP response header when the SSE stream is created and exposes it on ResponseStream. Fixture and websocket streams set the field to None because they do not have that HTTP response header.
• codex-core carries that stream-level id into InferenceTraceAttempt when recording terminal stream outcomes. Completed, failed, cancelled, dropped-stream, and pre-response error paths all record the id when it is available.
• rollout-trace now records both identifiers in raw terminal inference events and response payloads: response_id for the Responses API response.id, and upstream_request_id for x-request-id.
• The reducer stores both fields on InferenceCall. It also uses response_id for previous_response_id conversation linking, which removes the old accidental dependency on the misnamed upstream_request_id field.
• Terminal inference reduction now consumes the full terminal payload (InferenceCompleted, InferenceFailed, or InferenceCancelled) in one place. That keeps status, partial payloads, response ids, and upstream request ids consistent across success, failure, cancellation, and late stream-mapper events.

Why This Shape

x-request-id is a property of the HTTP/provider response envelope, not an SSE event. Capturing it once in codex-api and plumbing it through terminal trace recording avoids trying to infer the value from stream contents, and it preserves the id even when the stream fails or is cancelled after only partial output.

Keeping response_id separate from upstream_request_id also makes the reduced trace model less surprising: response_id remains the conversation-continuation id, while upstream_request_id is the operational correlation id for upstream debugging.

Validation

The PR updates trace and reducer coverage for:

• reading x-request-id from SSE response headers;
• storing the true upstream request id on completed inference calls;
• preserving upstream request ids for cancelled and late-cancelled inference streams;
• keeping previous_response_id reconstruction tied to response_id rather than transport request ids.

[cassirer-openai]

@codex review

[chatgpt-codex-connector]

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[jif-oai]

Beside my nit, lgtm