[Bugfix] Account for truncate_prompt_tokens when computing max_tokens

[viktorpusTT]

Purpose

When a request sets truncate_prompt_tokens, the prompt is truncated before inference but get_max_tokens() was receiving the original (untruncated) prompt length. This caused the output token budget to be underestimated by (original_len - truncated_len) tokens. Fix: compute input_length as min(prompt_len, truncate_prompt_tokens) before the get_max_tokens() call. Applies to both the /chat/completions and /completions endpoints. Add a focused unit test in test_serving_chat.py that covers the normal truncation case and the truncate_prompt_tokens=-1 sentinel.

Test Plan

Added dedicated test in tests/entrypoints/openai/chat_completion/test_serving_chat.py

Test Result

Pass

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[claude]

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[mergify]

Hi @viktorpusTT, the pre-commit checks have failed. Please run:

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[gemini-code-assist]

Code Review

This pull request ensures that max_tokens is calculated using the truncated prompt length when truncate_prompt_tokens is specified, preventing the output budget from being underestimated. It includes a new regression test and updates the logic in both chat and completion serving modules. The reviewer identified code duplication between these modules and suggested centralizing the truncation logic into a shared utility or base class for better maintainability, while also providing a more concise implementation.

[DarkLight1337]

This is also called in Responses API

[DarkLight1337]

Thanks