Wrap each request in a service scope (#276)

* Wrap each request in a service scope

* Change request/notification delegates to return ValueTask

* Address feedback

src/ModelContextProtocol/Protocol/Types/ServerCapabilities.cs

@@ -83,5 +83,5 @@ public class ServerCapabilities
     /// </para>
     /// </remarks>
     [JsonIgnore]
-    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, Task>>>? NotificationHandlers { get; set; }
+    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, ValueTask>>>? NotificationHandlers { get; set; }
 }