[gpt-oss] Convert user input to harmony format

[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

Convert user input to harmony format

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

Please merge it after #22393

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces support for the Harmony format, particularly for gpt-oss models, by adding logic to convert user input accordingly. The changes are extensive, touching on API server arguments, protocol definitions, and serving logic. I've identified a few critical issues: a bug in parsing reasoning messages that could lead to a server error, an in-place modification of cached conversation history which could cause inconsistent behavior, and an incomplete implementation of the --tool-server argument. Addressing these will improve the robustness and correctness of the new functionality.

[gemini-code-assist]

The implementation for the --tool-server argument is incomplete. It only handles the demo case, while the documentation in vllm/entrypoints/openai/cli_args.py states that it also accepts a comma-separated list of host:port pairs. This functionality is not implemented, which can be misleading for users as any value other than 'demo' will silently disable the tool server.

[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