Remove assert from Http3Connection.SendAsync (dotnet#74348)

* Remove assert in System.Net.Http.Http3Connection.SendAsync

* Minor change

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3Connection.cs

@@ -240,7 +240,7 @@ public async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, lon
             catch (QuicException ex) when (ex.QuicError == QuicError.OperationAborted)
             {
                 // This will happen if we aborted _connection somewhere and we have pending OpenOutboundStreamAsync call.
-                Debug.Assert(_abortException is not null);
+                // note that _abortException may be null if we closed the connection in response to a GOAWAY frame
                 throw new HttpRequestException(SR.net_http_client_execution_error, _abortException, RequestRetryType.RetryOnConnectionFailure);
             }
             finally
@@ -567,6 +567,10 @@ private async Task ProcessServerStreamAsync(QuicStream stream)
                     }
                 }
             }
+            catch (QuicException ex) when (ex.QuicError == QuicError.OperationAborted)
+            {
+                // ignore the exception, we have already closed the connection
+            }
             catch (Exception ex)
             {
                 Abort(ex);