fix(translation): preserve final answers in buffered Responses

[nachiketb-nvidia]

What

Normalize buffered OpenAI Responses output items into one semantic assistant output.

Why

first_output() means the first semantic ResponseOutput, not the first content block or modality. One ResponseOutput can contain ordered reasoning, text, tool calls, images, and other normalized content blocks. Chat, Anthropic, and accumulated streams already represent one completed assistant response this way.

The Responses wire format can instead split one logical completion across multiple top-level output[] items, such as a reasoning item followed by a message containing the visible final answer. The buffered decoder incorrectly mapped each wire item to a separate ResponseOutput. Anthropic and Chat then encoded the first semantic output, preserving its reasoning but never reaching the separately stored final message. Requests therefore returned HTTP 200 while silently dropping the visible answer.

This change normalizes the supported Responses output item types: reasoning, message, and function call. It does not broaden multimodal output support; audio, video, and unknown top-level Responses output items remain separate translation coverage gaps.

This fixes SWITCH-1187.

How

• Fold decoded Responses reasoning, message, and function-call items into ordered content blocks in one ResponseOutput.
• Retain the last meaningful role and stop reason.
• Preserve the existing empty-response fallback.
• Keep the Anthropic, Chat, and streaming production codecs unchanged.
• Verify buffered normalization matches the equivalent accumulated stream.

What to review

• Responses output-item ordering during normalization.
• Stop-reason selection across reasoning, message, and tool-call items.
• Buffered/stream semantic parity.

Live validation

Built the release server from this branch and ran the issue reproducer with live credentials:

• Direct Responses control: HTTP 200, reasoning,message, exact final-answer marker present.
• Chat through Switchyard: HTTP 200, reasoning non-empty, exact final-answer marker present.
• Anthropic through Switchyard: three consecutive HTTP 200 responses containing thinking,text, exact final-answer marker present.

One earlier Anthropic-format generation returned reasoning only. Since each call is a separate stochastic generation, it did not establish translation loss; every retry that produced final text preserved it.

Validation

• cargo fmt --all --check
• cargo test -p switchyard-translation
• cargo clippy -p switchyard-translation --all-targets -- -D warnings

[coderabbitai]

Walkthrough

decode_response now combines multiple OpenAI Responses output items into one ResponseOutput. Tests verify reasoning and assistant content across Anthropic Messages, OpenAI Chat, buffered decoding, and streamed events.

Changes

Responses output handling

Layer / File(s)	Summary
Buffered output aggregation and translation coverage crates/switchyard-translation/src/codecs/responses/buffered.rs, crates/switchyard-translation/tests/response_translation.rs	decode_response aggregates reasoning and assistant output. Translation tests verify separate Anthropic thinking and text blocks, plus OpenAI Chat reasoning and visible answer content.
Buffered and streamed output parity crates/switchyard-translation/tests/stream_translation.rs	A regression test verifies equivalent normalized output from buffered Responses decoding and streamed event accumulation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Poem

A rabbit watched the responses flow,
With thoughts kept private, answers shown.
One output gathered, neat and bright,
Stream and buffer now match just right.
“Hop!” said the rabbit, “tests take flight.”

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: preserving final answers in buffered OpenAI Responses translation.

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