[V1] Raise error if chunked prefill is explicitly disabled

[22quinn]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Currently when a user specify --no-enable-chunked-prefill or enable_chunked_prefill=False, it will silently overwrite the setting to True. This PR raises an explicit error telling user it's not supported.
Users have asked for this behaviour e.g. in #18547

Test Plan

vllm serve:

$ vllm serve Qwen/Qwen3-0.6B --no-enable-chunked-prefill
INFO 07-25 21:36:36 [__init__.py:235] Automatically detected platform cuda.
INFO 07-25 21:36:38 [api_server.py:1775] vLLM API server version 0.1.dev7998+g2f6e6b3
INFO 07-25 21:36:38 [cli_args.py:264] non-default args: {'model_tag': 'Qwen/Qwen3-0.6B', 'enable_chunked_prefill': False}
INFO 07-25 21:36:42 [config.py:1605] Using max model len 40960
Traceback (most recent call last):
--- stack omitted ---
  File "/home/user/dev/vllm/vllm/engine/arg_utils.py", line 1533, in _set_default_args_v1
    raise ValueError(
ValueError: Cannot disable chunked prefill for V1 engine.

LLM class:

>>> llm = LLM(model="Qwen/Qwen3-0.6B", enable_chunked_prefill=False)
INFO 07-25 21:37:26 [config.py:1605] Using max model len 40960
Traceback (most recent call last):
...
  File "/home/user/dev/vllm/vllm/engine/arg_utils.py", line 1533, in _set_default_args_v1
    raise ValueError("Cannot disable chunked prefill for V1 engine.")
ValueError: Cannot disable chunked prefill for V1 engine.

Test Result

See above

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request introduces a validation check to explicitly raise an error if a user attempts to disable chunked prefill for the V1 engine, which is an unsupported configuration. Previously, this setting was silently ignored and overridden. The change is straightforward, correctly implemented, and improves the user experience by providing clear feedback on unsupported configurations. The test cases provided in the pull request description adequately demonstrate the new behavior.

[22quinn]

@DarkLight1337 do you know the context for this? Feels we can set False individually for tests that need them?

[DarkLight1337]

I forgot, it has been so long ago

[DarkLight1337]

@WoosukKwon does it make sense to disable this completely?

[lhtin]

This error message is very helpful for the user experience. Any update?

[22quinn]

closing for #28833