[CI] Disable flaky structured output test from CI

[ywang96]

Purpose

v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-lm-format-enforcer-auto-None]
This particular test has been flaky on main CI for more than 2 weeks and has blocked auto-merge for quite a number of pull requests but it doesn't have an owner to fix. This PR disables it from main CI since this is a test on a particular model with a particular feature so it should be low risk.

Test Plan

Test Result

---

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.

[gemini-code-assist]

Code Review

This pull request disables a flaky test to unblock the CI pipeline. While this is a practical short-term solution, I've added a high-severity comment suggesting to create and link a tracking issue. This will ensure that the flaky test is not forgotten and is eventually fixed and re-enabled, which is crucial for maintaining long-term code quality and test coverage.

[houseroad]

Shall we create an issue to track the progress?

[houseroad]

Or we never want to bring it back :-)

[ywang96]

Or we never want to bring it back :-)

@aarnphm is already working on it - I just disabled it to unblock CI #24380