fixup! Improve exception handling

src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/MsQuicConnection.cs

@@ -590,6 +590,7 @@ private async ValueTask<QuicStreamProvider> OpenStreamAsync(QUIC_STREAM_OPEN_FLA
                 throw new InvalidOperationException(SR.net_quic_not_connected);
             }
 
+            cancellationToken.ThrowIfCancellationRequested();
             var stream = new MsQuicStream(_state, flags);
 
             try

src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/MsQuicStream.cs

@@ -894,7 +894,7 @@ internal async ValueTask StartAsync(QUIC_STREAM_START_FLAGS flags, CancellationT
             cancellationToken.ThrowIfCancellationRequested();
             using CancellationTokenRegistration registration = cancellationToken.UnsafeRegister(static (s, token) =>
             {
-                ((State)s!).StartCompletionSource.TrySetCanceled(token);
+                ((State)s!).StartCompletionSource.TrySetException(new OperationCanceledException(token));
             }, _state);
 
             uint status = MsQuicApi.Api.StreamStartDelegate(_state.Handle, flags);