[PD] Fix unclosed prefill connection warning of mini_lb (#5155)

Signed-off-by: Shangming Cai <caishangming@linux.alibaba.com>

python/sglang/srt/disaggregation/mini_lb.py

@@ -232,6 +232,9 @@ async def handle_generate_request(request_data: dict):
                     yield b"data: " + orjson.dumps(
                         error_msg, option=orjson.OPT_NON_STR_KEYS
                     ) + b"\n\n"
+                finally:
+                    if prefill_response is not None:
+                        await prefill_response.release()
 
         return StreamingResponse(
             stream_results(),