[gpt-oss] Add test for response API + harmony (but skipped)

[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

Add test for response API + harmony. Note that as main branch doesn't support gpt-oss, all tests are skipped for now. And the test is a bit weak. May need more help on improving coverage and adding stronger assertions. These tests should be run with

EXA_API_KEY=**** VLLM_ENABLE_RESPONSES_API_STORE=1 pytest -vs test_response_api_with_harmony.py

Test Plan

Run it locally

Test Result

Pass after #22512

(Optional) Documentation Update

[github-actions]

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[gemini-code-assist]

Code Review

This pull request adds a comprehensive test suite for the new response API with harmony. The tests are currently skipped. My review focuses on correctness. I found an issue in test_function_calling_full_history where the conversation history is built incorrectly, which would cause the test to fail. I've provided a suggestion to fix it.

[heheda12345]

Let's wait until entrypoint test is passed to ensure it doesn't break this test