fix: store session after handleRequest so sessionId is initialized

transport.sessionId is only populated during handleRequest(), not
before. Reading it before that call stored sessions under key
`undefined`, causing all subsequent requests to get "Session not found."

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/http-server.ts

@@ -134,14 +134,14 @@ const httpServer = http.createServer(async (req, res) => {
         const mcpServer = await createServer(config);
         await mcpServer.connect(transport);
 
+        await transport.handleRequest(req, res, parsedBody);
+
         const sid = transport.sessionId!;
         sessions.set(sid, { transport, createdAt: Date.now() });
 
         transport.onclose = () => {
           sessions.delete(sid);
         };
-
-        await transport.handleRequest(req, res, parsedBody);
       } else {
         // Existing session
         const session = sessions.get(sessionId);