[CI] xgrammar structured output supports Enum.

[chaunceyjiang]

xgrammar structured output supports Enum.

Fix: https://buildkite.com/vllm/ci/builds/16518#0195de7e-5b32-42e7-b741-8747cb4985db

Failure it was caused by #15594

[github-actions]

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[chaunceyjiang]

test

pytest tests/v1/structured_output/test_utils.py -s -v

[chaunceyjiang]

/cc @russellb @DarkLight1337 PTAL.