[gpt-oss] Support tool call and implement MCP tool server

[heheda12345]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Support tool call for gpt-oss model and implement MCP tool server

Test Plan

All tests in tests/v1/entrypoints/openai/responses to test whether it breaks existing model. Harmony integration is not tested yet.

Test Result

Passed

(Optional) Documentation Update

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces significant new functionality by adding support for tool calls for gpt-oss models and implementing an MCP (Model Component Protocol) tool server. The changes span across API protocols, serving logic, and utility functions. While the implementation is largely well-structured, I have identified a few critical issues that need to be addressed. These include an incorrect Pydantic model instantiation that will cause a runtime error, and a couple of bugs in the new MCPToolServer related to its class inheritance and method logic. Fixing these issues is crucial for the stability and correctness of the new features.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @heheda12345.

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