[Responses] Support required function tools for GPT-OSS Harmony

[ankrovv]

Summary

Adds GPT-OSS/Harmony Responses API support for tool_choice="required" when all tools are function tools.

This uses native JSON-schema constrained decoding for the required tool-call output. vLLM then converts the JSON into Responses function_call items. Streaming emits function-call argument events, not output text.

Scope:

• supports tool_choice="required" with function tools
• keeps existing auto and none behavior unchanged
• returns 400 for required hosted/non-function tools
• accepts text.format alongside required (silent precedence: tool-call schema wins this turn)
• returns 400 if structured_outputs is set alongside required
• does not use raw Harmony structural_tag

Reasoning behavior

The required path prefills <|channel|>final<|message|> so the JSON-schema constraint can apply cleanly from the first generated token. As a result the model emits the tool-call array directly and does not go through the analysis (reasoning) channel — response.output[] contains only function_call items, no ResponseReasoningItem. Preserving reasoning would require either raw Harmony structural_tag or a delayed-activation grammar primitive.

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[mergify]

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

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

[sfeng33]

Thanks for the work! FYI we have ongoing efforts to add structural tag support for gpt oss #45003. We can use this when it's required tool choice for gpt oss, which should be easier and more robust than the native structural output implementation.

[sfeng33]

Hey @ankrovv, could you try out #45560 and see if fixes the issue?