Wrap each request in a service scope (#276)

* Wrap each request in a service scope

* Change request/notification delegates to return ValueTask

* Address feedback

tests/ModelContextProtocol.AspNetCore.Tests/SseIntegrationTests.cs

@@ -87,7 +87,7 @@ public partial class SseIntegrationTests(ITestOutputHelper outputHelper) : Kestr
         {
             var msg = args.Params?["message"]?.GetValue<string>();
             receivedNotification.SetResult(msg);
-            return Task.CompletedTask;
+            return default;
         });
 
         // Send a test message through POST endpoint