refactor(link): bound every teardown and handler wait behind one helper [CI]

backend/shared/MoShiro.Measurement/Net/MsQuicConnection.cs

@@ -271,7 +271,15 @@ internal sealed class MsQuicConnection : IAsyncDisposable, IDatagramConnection
 
         if (Volatile.Read(ref _handle) != 0)
         {
-            await _shutdown.Task.ConfigureAwait(false);
+            // 原生层偶尔不回报关闭完成：等一会儿就放手，调用方（重拨循环、有序退出）不能跟着卡住
+            try
+            {
+                await _shutdown.Task.WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(false);
+            }
+            catch (TimeoutException)
+            {
+                _log.Warn("QUIC connection did not report shutdown completion in time; moving on");
+            }
         }
     }