[Bug]: Duplicate request_id breaks the engine

[tjohnson31415]

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🐛 Describe the bug

The request_id field used in the LLM Engine is assumed to be unique, but it is a parameter that can be set by the caller. After #9550, the request_id is configurable even by a caller to the OpenAI Server.

I originally encountered this issue while investigating the cause of negative metrics in #10430. I found that sending two requests concurrently with the same request id can cause the negative metric error. Example bash using curl:

curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
        "model": "meta-llama/Llama-3.2-3B-Instruct",
        "request_id": "id",
        "messages": [{"role": "user", "content": "Tell me a long story:"}],
        "max_tokens": 1024,
        "min_tokens": 1024,
        "temperature": 0
      }' &

sleep 0.5

curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
        "model": "meta-llama/Llama-3.2-3B-Instruct",
        "request_id": "id",
        "messages": [{"role": "user", "content": "Tell me a long story:"}],
        "max_tokens": 1024,
        "min_tokens": 1024,
        "temperature": 0
      }' &

The fix in #10430 properly prevents the crash, but only one request gets a response and the other connection hangs open.

Other users have reported other bad behavior that looks to be caused by duplicate request_ids:

• Offline server hangs: [Bug]: AsyncLLMEngine hangs when using asyncio.gather with vllm version 0.5.5 #8393
• "RuntimeError: aclose(): asynchronous generator is already running": [Bug]: RuntimeError: aclose(): asynchronous generator is already running #6325
• Counters can only be incremented by non-negative amounts.: [Bug]: error Counters can only be incremented by non-negative amounts. in metrics module #6642

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