fix(initialize scenario): return 202 No Content for notifications (closes #274)

The transport spec (2025-11-25 § 2.1, point 4) requires that a server
reply to a JSON-RPC notification with 202 Accepted and no body. The
fall-through branch in the initialize scenario test server returned
200 with `{"jsonrpc":"2.0","id":null,"result":{}}` for all non-initialize,
non-tools/list requests - including `notifications/initialized`.

Branch on `request.id == null` (notification) and emit 202 with no body
in that case. Non-notification requests still get the prior placeholder
200-response behavior.

Closes #274.

Author: Aditya-Chilka

src/scenarios/client/initialize.ts

@@ -78,6 +78,12 @@ export class InitializeScenario implements Scenario {
           this.handleInitialize(request, res);
         } else if (request.method === 'tools/list') {
           this.handleToolsList(request, res);
+        } else if (request.method === 'notifications/initialized') {
+          // Per MCP spec 2025-11-25 section 2.1 (Sending Messages to the Server),
+          // point 4: "If the input is a JSON-RPC response or notification ...
+          // the server MUST return HTTP status code 202 Accepted with no body."
+          res.writeHead(202);
+          res.end();
         } else {
           res.writeHead(200, { 'Content-Type': 'application/json' });
           res.end(