Replace Task.WhenAny + Task.Delay timeout pattern with .WaitAsync(TimeSpan) (#805)

* Initial plan

* Replace Task.WhenAny+Task.Delay with .WaitAsync(TimeSpan)

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

dotnet/test/MultiClientTests.cs

@@ -134,11 +134,9 @@ public async Task Both_Clients_See_Tool_Request_And_Completion_Events()
         Assert.Contains("MAGIC_hello_42", response!.Data.Content ?? string.Empty);
 
         // Wait for all broadcast events to arrive on both clients
-        var timeout = Task.Delay(TimeSpan.FromSeconds(10));
-        var allEvents = Task.WhenAll(
+        await Task.WhenAll(
             client1Requested.Task, client2Requested.Task,
-            client1Completed.Task, client2Completed.Task);
-        Assert.Equal(allEvents, await Task.WhenAny(allEvents, timeout));
+            client1Completed.Task, client2Completed.Task).WaitAsync(TimeSpan.FromSeconds(10));
 
         await session2.DisposeAsync();
 

dotnet/test/SessionTests.cs

@@ -272,8 +272,7 @@ public async Task Should_Receive_Session_Events()
         await session.SendAsync(new MessageOptions { Prompt = "What is 100+200?" });
 
         // Wait for session to become idle (indicating message processing is complete)
-        var completed = await Task.WhenAny(idleReceived.Task, Task.Delay(TimeSpan.FromSeconds(60)));
-        Assert.Equal(idleReceived.Task, completed);
+        await idleReceived.Task.WaitAsync(TimeSpan.FromSeconds(60));
 
         // Should have received multiple events (user message, assistant message, idle, etc.)
         Assert.NotEmpty(receivedEvents);