perf(tests): isolate network E2E and await QUIC cleanup

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

@@ -10,7 +10,7 @@ namespace MoShiro.Measurement;
 /// open one QUIC connection from a newly selected ephemeral source port. Measurement frames are sent
 /// only with MsQuic DATAGRAM calls; the existing TLS peer connection continues to carry session control.
 /// </summary>
-public sealed class QuicDatagramTransport : IDisposable
+public sealed class QuicDatagramTransport : IDisposable, IAsyncDisposable
 {
     private readonly MeasurementDatagramRouter _router;
     private readonly INodeLog _log;
@@ -166,6 +166,16 @@ public sealed class QuicDatagramTransport : IDisposable
         _ = DisposeNativeResourcesAsync();
     }
 
+    public async ValueTask DisposeAsync()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+        {
+            return;
+        }
+
+        await DisposeNativeResourcesAsync().ConfigureAwait(false);
+    }
+
     private async Task DisposeNativeResourcesAsync()
     {
         try