test(03-04): add failing tests for RemoteConfigStep

- BACK-01: Azure Blob form with account field + AzureAuthToggle
- BACK-02: S3 form fields (access_key_id, secret_access_key, region)
- BACK-03: S3-compatible form includes endpoint field
- renderWithBackend helper dispatches SET_BACKEND_TYPE before render
- @testing-library/user-event installed for value retention test

package-lock.json

@@ -18,6 +18,7 @@
         "@tailwindcss/vite": "^4.2.2",
         "@testing-library/dom": "^10.4.1",
         "@testing-library/react": "^16.3.2",
+        "@testing-library/user-event": "^14.6.1",
         "@types/react": "^18.3.5",
         "@types/react-dom": "^18.3.0",
         "@vitejs/plugin-react": "^4.7.0",
@@ -1797,6 +1798,20 @@
         }
       }
     },
+    "node_modules/@testing-library/user-event": {
+      "version": "14.6.1",
+      "resolved": "https://registry.npmjs.org/@testing-library/user-event/-/user-event-14.6.1.tgz",
+      "integrity": "sha512-vq7fv0rnt+QTXgPxr5Hjc210p6YKq2kmdziLgnsZGgLJ9e6VAShx1pACLuRjd/AS/sr7phAR58OIIpf0LlmQNw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=12",
+        "npm": ">=6"
+      },
+      "peerDependencies": {
+        "@testing-library/dom": ">=7.21.4"
+      }
+    },
     "node_modules/@types/aria-query": {
       "version": "5.0.4",
       "resolved": "https://registry.npmjs.org/@types/aria-query/-/aria-query-5.0.4.tgz",