[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
• does not use raw Harmony structural_tag
• does not change the reasoning gate

[github-actions]

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