Fix flaky OutgoingFilter_MultipleFilters_ExecuteInOrder test

The outermost filter's 'after' callback runs after the response has been
sent to the client via next(). In stateless/SSE mode, ListToolsAsync can
return before the server-side filter pipeline finishes executing the
'after' callbacks. Assert.Equal then enumerates the executionOrder list
concurrently with the filter adding to it, causing 'Collection was
modified' InvalidOperationException.

Fix by waiting for the outermost filter's 'after' callback to signal
completion before asserting on the execution order.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericstj

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -394,6 +394,7 @@ await client.CallToolAsync("sampling-tool",
     public async Task OutgoingFilter_MultipleFilters_ExecuteInOrder()
     {
         var executionOrder = new List<string>();
+        var allFiltersComplete = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
 
         Builder.Services.AddMcpServer()
             .WithHttpTransport(ConfigureStateless)
@@ -411,6 +412,7 @@ public async Task OutgoingFilter_MultipleFilters_ExecuteInOrder()
                     if (context.JsonRpcMessage is JsonRpcResponse r2 && r2.Result is JsonObject obj2 && obj2.ContainsKey("tools"))
                     {
                         executionOrder.Add("filter1-after");
+                        allFiltersComplete.TrySetResult();
                     }
                 });
 
@@ -439,6 +441,11 @@ public async Task OutgoingFilter_MultipleFilters_ExecuteInOrder()
 
         await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
 
+        // The outermost filter's "after" callback runs after the response has been
+        // sent to the client, so ListToolsAsync may return before it executes.
+        // Wait for it to complete before asserting.
+        await allFiltersComplete.Task.WaitAsync(TestContext.Current.CancellationToken);
+
         Assert.Equal(["filter1-before", "filter2-before", "filter2-after", "filter1-after"], executionOrder);
     }