feat(scenario): export/import endpoints and integration tests

- add GET /scenarios/:id/export returning ScenarioExportDto
- add POST /scenarios/import creating scenario with all steps
- add ScenarioExportDto and ScenarioStepExportDto classes
- fix McpServer singleton by creating fresh instance per handle() call
- add 12 integration tests covering shape, ordering, round-trip, validation

src/mcp/mcp.service.ts

@@ -15,8 +15,6 @@ import pkg from '../../package.json';
 
 @Injectable()
 export class McpService {
-  private readonly server: McpServer;
-
   constructor(
     private readonly authService: AuthService,
     private readonly sessionService: SessionService,
@@ -24,13 +22,15 @@ export class McpService {
     private readonly browserService: BrowserService,
     private readonly codeExecutor: CodeExecutorService,
     private readonly scenarioService: ScenarioService,
-  ) {
-    this.server = new McpServer({ name: pkg.name, version: pkg.version });
-    this.registerTools();
+  ) {}
+
+  private createServer(): McpServer {
+    const server = new McpServer({ name: pkg.name, version: pkg.version });
+    this.registerTools(server);
+    return server;
   }
 
-  private registerTools(): void {
-    const server = this.server;
+  private registerTools(server: McpServer): void {
 
     // ── Auth ──────────────────────────────────────────────────────────────────
 
@@ -454,12 +454,14 @@ export class McpService {
   }
 
   async handle(req: Request, res: Response): Promise<void> {
+    const server = this.createServer();
     const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: undefined });
-    await this.server.connect(transport);
+    await server.connect(transport);
     try {
       await transport.handleRequest(req, res, req.body);
     } finally {
       await transport.close();
+      await server.close();
     }
   }
 }