feat(server): add serve log-level flag

[XiaoPengMei]

Closes #50

Summary

• add --log-level to both rapid-mlx serve and python -m vllm_mlx.server
• apply the selected level to Python logging and pass the normalized value through to uvicorn.run
• add focused tests that verify both entrypoints expose the new flag

Testing

• pytest tests/test_harmony_parsers.py -k log_level
• manual QA: invoked serve_command() with --log-level WARNING using a stubbed server and verified uvicorn.run(..., log_level='warning') plus root logger level 30

[raullenchai]

Thanks for the PR, @XiaoPengMei! The feature itself is straightforward and welcome. A few things to address before merge:

Issues

P1: Case-insensitive input rejected

choices=["DEBUG", "INFO", "WARNING", "ERROR"] means --log-level debug (lowercase) is rejected by argparse. Most CLI tools accept either case. Fix: add type=str.upper to the argument so any casing is accepted:

serve_parser.add_argument(
    "--log-level",
    type=str.upper,
    choices=["DEBUG", "INFO", "WARNING", "ERROR"],
    default="INFO",
    ...
)

This also makes normalize_log_level() unnecessary — argparse handles it.

P2: Root logger modification is too broad

logging.getLogger().setLevel(...) changes the root logger, which affects every library (httpx, uvicorn internals, asyncio, etc.). At DEBUG this floods output with noise unrelated to vllm-mlx.

Instead, only set the vllm_mlx logger hierarchy:

def configure_logging(log_level: str) -> str:
    level = getattr(logging, log_level, logging.INFO)
    logging.getLogger("vllm_mlx").setLevel(level)
    return log_level.lower()  # uvicorn wants lowercase

P3: Tests are source-code grep, not behavioral

The tests inspect source code strings ('"--log-level"' in source). This passes even if the flag is broken at runtime. A better approach would be to actually parse args through argparse and verify the result. Also, these tests belong in a CLI/server test file, not test_harmony_parsers.py.

Minor

• normalize_log_level() is just .upper() — can be removed if you use type=str.upper in argparse.
• Missing blank line between configure_logging() and # Global engine instance comment.