fix(translation): honor Chat streaming usage opt-in

[ajcasagrande]

What

Makes generated OpenAI Chat streaming usage opt-in through stream_options.include_usage. Requested usage now follows the finish chunk as one choices: [] frame at clean EOF. The relay test confirms hidden client output still emits answer-token telemetry.

Why

OpenAI Chat callers that omit usage opt-in must not receive generated usage frames. This keeps provider raw replay unchanged while preserving clean-EOF ordering and telemetry behavior.

Notes for reviewers

• RED to GREEN contract tests cover omitted, false, non-boolean, true, missing backend usage, post-stop errors, and raw replay.
• Full workspace tests passed. Focused relay tests, relay crate tests, scoped Clippy, formatting, and diff checks passed.
• Fresh Graham review approved with zero findings.

Summary by CodeRabbit

• New Features

  • OpenAI Chat streaming responses can now include a separate final usage-only chunk when explicitly requested.
  • Usage information is emitted after normal completion and remains available even when no backend usage data is returned.

• Bug Fixes

  • Prevented usage data from being embedded in terminal message chunks.
  • Usage is no longer emitted for errored streams or when usage inclusion is not requested.
  • Ensured usage is emitted at most once and stream replay reaches the correct final state.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository: NVIDIA-NeMo/Switchyard/.coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 8e6049e3-9f2c-4c7d-9b32-5fb383c5a2fc

📥 Commits

Reviewing files that changed from the base of the PR and between bfcd023 and 71ae90a.

📒 Files selected for processing (7)

• crates/switchyard-nemo-relay-plugin/src/runtime.rs
• crates/switchyard-server/src/lib.rs
• crates/switchyard-translation/src/codecs/openai_chat/stream.rs
• crates/switchyard-translation/src/codecs/stream.rs
• crates/switchyard-translation/src/helpers.rs
• crates/switchyard-translation/tests/response_translation.rs
• crates/switchyard-translation/tests/stream_translation.rs

Included review availability: Your plan provides up to 12 included reviews per hour; 9 remain after this review.

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Walkthrough

Changes

OpenAI Chat streaming usage

Layer / File(s)	Summary
Usage opt-in state and initialization crates/switchyard-translation/src/codecs/stream.rs, crates/switchyard-translation/src/helpers.rs, crates/switchyard-server/src/lib.rs	Stream state records Chat usage opt-in and finalization. Usage generation requires stream_options.include_usage: true.
Chat stream replay and finalization crates/switchyard-translation/src/codecs/openai_chat/stream.rs	Replay marks stopped streams as finalized. Usage updates state without immediate output. Successful finalization emits terminal and optional usage chunks separately.
Usage behavior validation crates/switchyard-translation/helpers.rs, crates/switchyard-translation/tests/*, crates/switchyard-nemo-relay-plugin/src/runtime.rs	Tests cover opt-in, replay, errors, state-only usage, and updated stream termination behavior.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~30 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 61.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: honoring the OpenAI Chat streaming usage opt-in during translation.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

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_{A rabbit checks the streaming trail
Usage waits beyond the final tale
Opt-in opens one last small door
Errors leave no usage on the floor
Replay keeps the stream in score}

_{Comment @coderabbitai help to get the list of available commands.}

[ajcasagrande]

closing for now, while product alignment is conferred