Centralize test timeout constants to fix CI flakiness (#1224)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ericstj <8918108+ericstj@users.noreply.github.com>
Co-authored-by: Eric StJohn <ericstj@microsoft.com>

tests/ModelContextProtocol.AspNetCore.Tests/Utils/KestrelInMemoryTest.cs

@@ -42,7 +42,7 @@ public class KestrelInMemoryTest : LoggedTest
     protected static void ConfigureHttpClient(HttpClient httpClient)
     {
         httpClient.BaseAddress = new Uri("http://localhost:5000/");
-        httpClient.Timeout = TimeSpan.FromSeconds(10);
+        httpClient.Timeout = TestConstants.HttpClientTimeout;
     }
 
     public override void Dispose()