Updated xgrammar backend to not deny supported string formats

[ExtReMLapin]

Purpose

Fixing string formats when using xgrammar, xgrammar is required when we want to prevent infinite generation (reasoning) with --structured-outputs-config "{\"disable_any_whitespace\": true, \"backend\": \"xgrammar\"}"

Source of the supported formats :
https://github.com/mlc-ai/xgrammar/blob/a32ac892676d2eedc0327416105b9b06edfb94b2/cpp/json_schema_converter.cc#L2992

Test Plan

updated tests to not deny 'email' format (it's supported) and to check if a non existing string is still denied.

Test Result

/opt/vllm$ CUDA_VISIBLE_DEVICES=-1 pytest vllm_last/tests/v1/structured_output/test_utils.py 
================================================================================= test session starts =================================================================================
platform linux -- Python 3.10.12, pytest-8.4.2, pluggy-1.6.0
rootdir: /opt/vllm/vllm_last
configfile: pyproject.toml
plugins: anyio-4.11.0
collected 6 items                                                                                                                                                                     

vllm_last/tests/v1/structured_output/test_utils.py ......                                                                                                                       [100%]

================================================================================== warnings summary ===================================================================================
<frozen importlib._bootstrap>:241
  <frozen importlib._bootstrap>:241: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:241
  <frozen importlib._bootstrap>:241: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================================ 6 passed, 2 warnings in 1.27s ============================================================================
sys:1: DeprecationWarning: builtin type swigvarlink has no __module__ attribute

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request updates the xgrammar backend to correctly handle supported string formats, which were previously being denied. The changes involve adding a list of supported formats and modifying the validation logic to check against this list. The tests have also been updated to reflect this change. My review focuses on improving the maintainability of the new code by adhering to Python's PEP 8 style guide for constants and by using a permanent link for the source reference. These changes will make the code more robust and easier to maintain in the future.

[ExtReMLapin]

cc main contributors of the backend :

@russellb @benchislett @shen-shanshan

[shen-shanshan]

LGTM if the test passed.

[russellb]

lgtm, thanks!