Improve literal dataclass field conversion to argparse argument

[hmellor]

• Enforces that Literals are merged like Literal[Literal[1], Literal[2]] instead of Union[Literal[1], Literal[2]] (this stops us from needing complicated Literal merging in get_kwargs)
• Handle list[Literal] where we want the argument to accept multiple values from a set of choices
• Adds tests for both ofd the above changes

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[Copilot]

Pull Request Overview

This PR refactors the conversion of Literal type hints for argparse by introducing a helper function (literal_to_kwargs) to centralize the logic. It also enforces that fields using Literals must merge nested literal values (e.g. Literal[Literal[1], Literal[2]] rather than Union[Literal[1], Literal[2]]), and adds tests to validate these behaviors.

• Consolidates Literal-to-kwargs conversion into a single helper function.
• Enforces stricter validation for Literal types in dataclass field definitions.
• Adds test cases to ensure proper handling of Literal and list[Literal] types.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
vllm/engine/arg_utils.py	Introduces literal_to_kwargs and updates get_kwargs accordingly.
vllm/config.py	Updates field validation to mandate merged Literal types.
tests/test_config.py	Adds tests to check proper configuration and error messages.
tests/engine/test_arg_utils.py	Adds tests for literal_to_kwargs behavior.

[russellb]

thanks!