[codex] emit per-request TTFT completion telemetry

[xli-oai]

Why

Codex telemetry pipeline needs a per-request TTFT value. The existing codex.turn_ttft is recorded once per turn, so it cannot represent later inference requests in the same turn and can miss the beginning of hidden reasoning.

This restores the low-volume per-request signal proposed in bk-nvidia#3 without bringing back per-WebSocket-event TRACE logging.

What changed

• start a timer when each mapped Responses stream begins
• latch the timer on the first response.output_item.added, including an empty hidden-reasoning item
• attach ttft_ms to the existing codex.sse_event / response.completed telemetry record
• cover the new completion field with an integration test

Semantics

The value is per inference request, not per turn. It measures mapped-stream-to-first-output-item latency, matching the customer-proposed metric. For HTTP, the stream is already established before timing begins, so request setup and response-header latency are excluded.

response.output_item.added is a client-visible proxy for the start of hidden reasoning; this does not claim access to the server's internal first raw-token timestamp.

Validation

• just test -p codex-otel (47 passed)
• just test -p codex-core process_sse_emits_completed_telemetry (1 passed after the final timer-placement change)
• attempted just test -p codex-core: 2,855 passed and 53 failed because of unrelated local-environment failures (missing test_stdio_server fixture binary, shell startup noise, and timing-sensitive tests); the focused telemetry test passed in that run as well

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 36dadfb8bc

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".