[Bugfix] Fix OpenAI parallel sampling when using xgrammar

[mgoin]

The xgrammar logit processor has internal state per request, so for parallel sampling we need to deepcopy the processor. This was not being handled correctly through the OpenAI API, so this PR adds that fix and updates tests/entrypoints/openai/test_completion.py to test this.

[github-actions]

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[DarkLight1337]

Looks reasonable, thanks for the fix!