fix(quic): exclude locally rejected datagrams [CI]

backend/shared/MoShiro.Measurement/Blocks/Train.cs

@@ -1,5 +1,6 @@
 using System.Diagnostics;
 using System.Net;
+using Fleet.Core;
 using MoShiro.Protocol;
 
 namespace MoShiro.Measurement;
@@ -121,7 +122,13 @@ public sealed class TrainListener : BlockBase
             {
                 long sendUs = Env.Clock.NowUs();
                 MeasurementDatagram.Write(packet, new MeasurementDatagramHeader(MeasurementDatagramType.Data, session.SessionId, Spec.StreamId, seq, sendUs), session.Key);
-                Env.Datagrams!.Send(session, target, packet);
+                if (!Env.Datagrams!.Send(session, target, packet))
+                {
+                    Env.Log.Warn("QUIC train datagram was not queued; stopping the train without counting it as loss",
+                        ("stream", Spec.StreamId), ("seq", seq), ("bytes", size));
+                    return;
+                }
+
                 Span<long> row = [size, sendUs];
                 _records.SetRow(seq, row);
                 Interlocked.Exchange(ref _sent, seq + 1L);
@@ -171,7 +178,10 @@ public sealed class TrainDialer : BlockBase
             for (uint seq = 0; seq < 3; seq++)
             {
                 MeasurementDatagram.Write(hello, new MeasurementDatagramHeader(MeasurementDatagramType.Data, session.SessionId, Spec.StreamId, seq, Env.Clock.NowUs()), session.Key);
-                datagrams.Send(session, target, hello);
+                if (!datagrams.Send(session, target, hello))
+                {
+                    throw new BlockFaultException($"QUIC datagram send failed during path discovery (seq {seq}).");
+                }
             }
 
             await WaitUntilCancelledAsync(ct);