[Bugfix] Fix --config arg expansion called from api_server.py

[dubejf]

Purpose

Per --help, the argument parser for the vllm CLI and the api_server entry point both accept the --config command-line argument, but only the CLI handles it correctly. This PR ensures consistent config file handling for all entry points.

Examples:

• vllm serve --config config.yaml
  ✅ Works as expected

• docker run <docker_arguments> vllm/vllm-openai --config config.yaml
  ❌ api_server.py: error: argument --config: expected one argument

• python3 -m vllm.entrypoints.openai.api_server --config config.yaml
  ❌ api_server.py: error: argument --config: expected one argument

Cause:
_pull_args_from_config expects the first argument to always be a vllm CLI subcommand (serve, chat, complete, etc.), which is not the case for api_server.

This leads to incorrect expansion:

--config config.yaml <other_cli_args>

is mistakenly expanded to:

--config --arg1 value-from-config --arg2 value-from-config ... <other_cli_args>

instead of:

--arg1 value-from-config --arg2 value-from-config ... <other_cli_args>

Test Plan

• Added unit tests for --config argument parsing in tests/entrypoints/openai/test_cli_args.py.

Test Result

• The new unit tests pass.
• Existing tests for CLI config override (in tests/utils_/test_utils.py) already cover configuration file expansion in the vllm serve context; no changes required.

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• [ x] The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug where the --config command-line argument was not properly handled when using the api_server.py entrypoint directly. The root cause was correctly identified: the argument parser expected a subcommand which is not always present. The fix introduces a new conditional branch to handle this specific case by checking if the first argument is an option. This change is well-contained and effective. Additionally, new unit tests have been added to cover the fixed behavior, including argument overrides, which strengthens the codebase. The overall change is solid and well-implemented.

[DarkLight1337]

Thanks for fixing!