Update test expectations for session cancellation with flushed responses

Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -79,7 +79,15 @@ public async ValueTask<bool> HandlePostAsync(JsonRpcMessage message, Cancellatio
                 // If there's no priming write, flush the stream to ensure HTTP response headers are
                 // sent to the client now that the server is ready to process the request.
                 // This prevents HttpClient timeout for long-running requests.
-                await responseStream.FlushAsync(cancellationToken).ConfigureAwait(false);
+                try
+                {
+                    await responseStream.FlushAsync(cancellationToken).ConfigureAwait(false);
+                }
+                catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+                {
+                    // The session was canceled before we could flush. This is expected during session disposal.
+                    // Don't propagate the exception - let the request handling continue and fail naturally.
+                }
             }
 
             // Ensure that we've sent the priming event before processing the incoming request.

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs

@@ -371,14 +371,14 @@ Task<HttpResponseMessage> CallLongRunningToolAsync() =>
         var sseResponseBody = await getResponse.Content.ReadAsStringAsync(TestContext.Current.CancellationToken);
         Assert.Empty(sseResponseBody);
 
-        // Currently, the OCE thrown by the canceled session is unhandled and turned into a 500 error by Kestrel.
+        // Currently, responses are flushed immediately to prevent HttpClient timeouts for long-running requests.
+        // This means the response starts with a 200 status code. When the session is canceled, the connection
+        // is closed abruptly, causing HttpClient errors.
         // The spec suggests sending CancelledNotifications. That would be good, but we can do that later.
-        // For now, the important thing is that request completes without indicating success.
-        await Task.WhenAll(longRunningToolTasks);
+        // For now, the important thing is that requests fail to complete successfully.
         foreach (var task in longRunningToolTasks)
         {
-            var response = await task;
-            Assert.False(response.IsSuccessStatusCode);
+            await Assert.ThrowsAnyAsync<HttpRequestException>(async () => await task);
         }
     }