Add allowedDomains to astro-5 test app config

Astro 5.17.2 changed host header validation to fall back to "localhost"
(without port) when no `allowedDomains` is configured, breaking internal
fetch requests in SSR pages. This caused the nested SSR route E2E tests
to time out waiting for the API endpoint transaction.

Author: andreiborza

dev-packages/e2e-tests/test-applications/astro-5/astro.config.mjs

@@ -15,6 +15,9 @@ export default defineConfig({
     }),
   ],
   output: 'server',
+  security: {
+    allowedDomains: [{ hostname: 'localhost' }],
+  },
   adapter: node({
     mode: 'standalone',
   }),