[vllm] Fix CLI argument serialization for list types

[jreiml]

What does this PR do?

Fixes CLI argument serialization for list-type config values in async vLLM server. Lists like cudagraph_capture_sizes=[1, 2, 4, 8] are now correctly expanded to separate CLI arguments.

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Test

Added CPU unit tests in tests/workers/rollout/test_vllm_cli_args_on_cpu.py covering:

• String, int, float values → ['--key', 'value']
• Bool True → ['--key'], Bool False → skipped
• None values → skipped
• List expansion → ['--key', '1', '2', '3']
• Empty lists → skipped (vLLM nargs="+" requires at least one value)
• Dict values → JSON serialized
• Mixed configs, order preservation, edge cases

API and Usage Example

No API changes.

Design & Code Changes

Problem:
When passing list-type configuration values (e.g., cudagraph_capture_sizes) to the vLLM async server, the old code used str(list) which produces "[1, 2, 4, 8]" as a single argument.

However, vLLM's argparse uses nargs="+" for list arguments, which expects multiple space-separated values like --cudagraph-capture-sizes 1 2 4 8.

This caused argparse to fail with: invalid int value: '[1, 2, 4, 8]'

Solution:

1. Extract build_cli_args_from_config() helper function for testability
2. Expand list values into multiple separate CLI arguments:

# Before: ['--cudagraph-capture-sizes', '[1, 2, 4, 8]']  # FAILS
# After:  ['--cudagraph-capture-sizes', '1', '2', '4', '8']  # WORKS

3. Handle edge case of empty lists (skip them since nargs="+" requires at least one value)

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue with serializing list-type CLI arguments for the vLLM async server. The changes ensure that lists are expanded into space-separated values as expected by argparse. I've included one suggestion to improve the code structure for better readability and maintainability.