fix: prevent QUIC heartbeat stalls and isolate TCP retransmission counters [CI] [SKIP TESTS]

backend/tests/MoShiro.TestSupport/AgentSimulator.cs

@@ -274,6 +274,11 @@ public sealed class AgentSimulator : IAsyncDisposable
             {
                 // 已经断开
             }
+            catch (OperationCanceledException) when (_stop.IsCancellationRequested)
+            {
+                // Canceling the receive loop may abort/dispose the transport
+                // between the State check and CloseOutputAsync.
+            }
         }
 
         if (_loop is not null)