[WIP][V1][Structured Output] Enable structured output test

[markmc]

See #14512 (comment)

Enable tests/v1/entrypoints/llm/test_struct_output_generate.py in CI.

WIP while we debug why this somethimes fails with:

RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method

The failure can be reproduced in some environments with:

VLLM_USE_V1=1 pytest -s -v 'tests/v1/entrypoints/llm/test_struct_output_generate.py::test_guided_grammar_ebnf[xgrammar]' 'tests/v1/entrypoints/llm/test_struct_output_generate.py::test_guided_grammar_lark[xgrammar]'

[github-actions]

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[markmc]

Sure enough, v1-test failed with:

[2025-03-10T22:48:18Z] ERROR 03-10 22:48:18 [core.py:324]   File "/opt/venv/lib/python3.12/site-packages/torch/cuda/__init__.py", line 305, in _lazy_init
[2025-03-10T22:48:18Z] ERROR 03-10 22:48:18 [core.py:324]     raise RuntimeError(
[2025-03-10T22:48:18Z] ERROR 03-10 22:48:18 [core.py:324] RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method
[2025-03-10T22:48:18Z] ERROR 03-10 22:48:18 [core.py:324]

[markmc]

Closing in favor of #14619