Apply code review feedback: inline duration calculation and remove logging test

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -357,8 +357,7 @@ private void HandleMessageWithId(JsonRpcMessage message, JsonRpcMessageWithId me
         LogRequestHandlerCalled(EndpointName, request.Method);
         long startingTimestamp = Stopwatch.GetTimestamp();
         JsonNode? result = await handler(request, cancellationToken).ConfigureAwait(false);
-        TimeSpan elapsed = GetElapsed(startingTimestamp);
-        LogRequestHandlerCompleted(EndpointName, request.Method, elapsed.TotalMilliseconds);
+        LogRequestHandlerCompleted(EndpointName, request.Method, GetElapsed(startingTimestamp).TotalMilliseconds);
 
         await SendMessageAsync(new JsonRpcResponse
         {