[Bug]: Curl failed: Received HTTP/0.9 when not allowed #20119

Your output of `python collect_env.py` here
🐛 Describe the bug
Issue Description:

In a data-parallel scenario, when the --data-parallel-rpc-port argument value conflicts with the vLLM HTTP service port, the service starts without any error logs despite the port collision.




    

Actual Behavior:

When clients send requests, they only receive the error message: "Received HTTP/0.9 when not allowed".
Expected Behavior:

The vLLM service should either:
Detect the port conflict during startup and fail with a clear error message, or

Dynamically handle the conflict by selecting an alternative port.
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