[CI][V1] Fix passing tokenizer as kwarg to validate_guidance_grammar

[ywang96]

llguidance.LLMatcher.validate_grammar no longer takes tokenizer as a keyword argument (not sure if this is a bug on their side), but this has caused V1 test to always break on main.

This PR fixes it.

tests/v1/entrypoints/llm/test_struct_output_generate.py ....                                                                                                           [100%]

============================================================================== warnings summary ==============================================================================
tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-xgrammar:disable-any-whitespace-auto]
tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-guidance:disable-any-whitespace-auto]
  /home/jovyan/vllm/vllm/transformers_utils/tokenizer_group/tokenizer_group.py:25: FutureWarning: It is strongly recommended to run mistral models with `--tokenizer-mode "mistral"` to ensure correct encoding and decoding.
    self.tokenizer = get_tokenizer(self.tokenizer_id, **tokenizer_config)

tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-xgrammar:disable-any-whitespace-auto]
tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-guidance:disable-any-whitespace-auto]
tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[mistralai/Ministral-8B-Instruct-2410-xgrammar:disable-any-whitespace-mistral]
tests/v1/entrypoints/llm/test_struct_output_generate.py::test_structured_output[Qwen/Qwen2.5-1.5B-Instruct-xgrammar:disable-any-whitespace-auto]
  /home/jovyan/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=132348) is multi-threaded, use of fork() may lead to deadlocks in the child.
    self.pid = os.fork()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================================================== 4 passed, 6 warnings in 75.87s (0:01:15) ==================================================================

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[mgoin]

LGTM