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

backend/probe/src/MoShiro.Probe/Link/AgentConnection.cs

@@ -196,7 +196,7 @@ public sealed class AgentConnection : IDisposable
         var remote = (IPEndPoint)info.Remote!;
         IReadOnlyList<int> quicPorts = Endpoint.EffectiveQuicPorts;
         int quicPort = quicPorts.Count == 0 ? 7443 : quicPorts[0];
-        using var datagrams = new QuicDatagramTransport(
+        await using var datagrams = new QuicDatagramTransport(
             data,
             _env.Clock,
             new DataPlaneCounters(),