Motivation

Improvement of sgl-router to avoid hang.

Modifications

• Fix sgl-router side of http payload handling to drain the payload when
  we hit early error. This covers the scenario of 404, and 413 large
  payload for json scenario.
• Align sglang client/server keepalive expectations to avoid sgl-router client hang

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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