[Feature]: Support stream in the vllm PD router

[cn-cjy]

🚀 The feature, motivation and pitch

While I using docker to deploy a 2P2D node + vllm-router on a physical machine to run a benchmark test, I found it didn't work and showed 【'Connection aborted.', RemoteDisconnected('Remote end closed connection without response')】. So I tried to use some AI Coding tools to figure out and solve it, it has been solved and it works.I'll submit it right away.

Source code:

After change:

Alternatives

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Additional context

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[cn-cjy]

I found the same code logic around lines 991–1007 in the "process_vllm_two_stage_request" method, so the issue should only exist in the "process_vllm_two_stage_request_discovered" method.