Add missing /api/exists route registration

The exists() endpoint handler was implemented but the route was never
registered in the container's router setup, causing e2e tests to fail
with HTTP 500 errors.

This adds the missing route registration for POST /api/exists to the
file handler in routes/setup.ts.

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

Author: github-actions[bot]

packages/sandbox-container/src/routes/setup.ts

@@ -91,6 +91,13 @@ export function setupRoutes(router: Router, container: Container): void {
     middleware: [container.get('loggingMiddleware')],
   });
 
+  router.register({
+    method: 'POST',
+    path: '/api/exists',
+    handler: async (req, ctx) => container.get('fileHandler').handle(req, ctx),
+    middleware: [container.get('loggingMiddleware')],
+  });
+
   // Port management routes
   router.register({
     method: 'POST',