[Frontend] Add structured_outputs field to ResponsesRequest

[MarcinGodniak]

Purpose

Expose structured_outputs as a field on ResponsesRequest to allow internal components (e.g., tool_parser) to modify structured output parameters during request processing.

Previously, structured_outputs was a local variable in to_sampling_params(), making it inaccessible to other parts of the request pipeline. This change promotes it to an instance field so it can be set/modified by tool parsers before sampling params are generated.

This aligns the Responses API with the existing behavior in /chat/completions, where structured_outputs is already available as a field.

Test Plan

pytest tests/entrypoints/openai/responses -v

Test Result

========================================================= warnings summary ==========================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

.venv/lib/python3.12/site-packages/schemathesis/generation/coverage.py:305
  /root/vllm_code/.venv/lib/python3.12/site-packages/schemathesis/generation/coverage.py:305: DeprecationWarning: jsonschema.exceptions.RefResolutionError is deprecated as of version 4.18.0. If you wish to catch potential reference resolution errors, directly catch referencing.exceptions.Unresolvable.
    ref_error: type[Exception] = jsonschema.RefResolutionError,

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================= 61 passed, 1 skipped, 3 warnings in 639.80s (0:10:39) =======================================

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[gemini-code-assist]

Code Review

This pull request adds the structured_outputs field to ResponsesRequest, aligning it with the ChatCompletionRequest and allowing for more flexible structured output configurations. The implementation in to_sampling_params correctly uses this new field. However, it introduces a side effect by mutating the request object. I've provided a suggestion to refactor this to avoid mutation, which improves code clarity and maintainability by making the method's behavior more predictable.

[chaunceyjiang]

Do you need to use this field?

[MarcinGodniak]

My ultimate goal is to support providing grammar via the tool parser.
For chat/completions requests, this field is used to analyze the request and, when necessary, populate the StructuredOutputsParams.grammar field, which is later consumed by to_sampling_params. I already use this field in chat/completions, and it seemed natural to replicate the same approach here.

I’m open to other approaches if there’s a better way to achieve this.

[chaunceyjiang]

For structured outputs, the Responses API has its own dedicated fields.

https://platform.openai.com/docs/guides/structured-outputs

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @MarcinGodniak.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork