[GPT-OSS] Structure_Tag support for gpt-oss tool-call in cot

[Hanchenli]

Purpose

The model sometimes generates function calls in CoT that have not been provided in tool_server. This PR uses structural tag to force model to 1. do not generate function calls when tool_server is not provided. 2. Only generate function that are present in the tool_server.

example custom structural tags: https://gist.github.com/frank-wei/4929118fe0881ee561ce48f15c1f80ed

Test Plan

Testing with @frank-wei

Test Result

vllm serve run: https://gist.github.com/frank-wei/86d37061685fd45e5b831b6583d865d6

see number of turns reduced:

• without structural tags: https://gist.github.com/frank-wei/3c89e45e028cb0f61dc56c51f464252f
• with structural tags: https://gist.github.com/frank-wei/45daec7615f0c75ab3d97594746324c2

---

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.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

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

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

[gemini-code-assist]

Code Review

This pull request introduces support for structural tags in gpt-oss tool calls, aiming to constrain the model's output to valid function calls. The changes span model configuration, engine arguments, the OpenAI protocol, and a new reasoning parser for gpt-oss. While the overall approach is sound, I've identified a critical bug in gptoss_reasoning_parser.py involving the mutation of a global dictionary, which could lead to incorrect behavior in subsequent requests. Additionally, there's a high-severity bug in a log statement within the same file that could cause runtime errors. I have provided detailed comments and code suggestions to address these issues. The rest of the changes, including new tests and dependency updates, appear to be in good order.

[frank-wei]

Looks like it is failed in trunk https://buildkite.com/vllm/ci/builds/35163/steps/canvas?sid=0199eb2d-ccad-4e6b-963d-ef868491a020?
Got this link from: https://app.hex.tech/533fe68e-dcd8-4a52-a101-aefba762f581/app/vLLM-CI-030kdEgDv6lSlh1UPYOkWP/latest?selectedStaticCellId=0199ac4b-8cf6-7ee8-8db1-2dff4166cbcf

[hmellor]

Yes this Phi3 test has been unreliable on main for a while. cc @DarkLight1337

[DarkLight1337]

I should have fixed it on main already. Let me rebase this PR