fix: Default max tokens to None for OpenAI frontend.

[thealmightygrant]

What does the PR do?

As described in #7796, the default for max_tokens should be None rather than 16. This is consistent with both OpenAI's endpoints and vLLM. Please see the linked issue for more details.

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• build
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Test plan:

I will test on my local triton instance that is running this frontend.

Caveats:

I didn't go through and try to fix any tests that assume the existing default.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: Problems with the response of the OpenAI-Compatible Frontend for Triton Inference Server #7796