[Misc][gpt-oss] Add rules to label gpt-oss related PRs

[draftbk]

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

Add rules to label gpt-oss related PRs

Test Plan

Test Result

[github-actions]

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To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request adds a new mergify rule to automatically label pull requests related to gpt-oss. The rule conditions look good for matching PR titles and some file paths. However, the file path matching conditions are a bit too restrictive and might miss some relevant files. For example, a file like vllm/reasoning/gptoss_reasoning_parser.py would not be matched by the current rules. I've suggested a change to make the regex more flexible to catch variations like gpt_oss, gpt-oss, and gptoss, and to make it case-insensitive.

[DarkLight1337]

LGTM, cc @simon-mo @WoosukKwon I have added gpt-oss label to GH already