[CI] Disable flaky structure decoding test temporarily.

[ywang96]

v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[Qwen/Qwen2.5-1.5B-Instruct-guidance:disable-any-whitespace-auto] has been flaky on CI and therefore blocking quite a few PRs.

This PR disables this test temporarily to unblock other folks - see discussion https://vllm-dev.slack.com/archives/C07R5PAL2L9/p1743492042284569

cc @russellb @mgoin

[github-actions]

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