CustomerError: Can't find required-server-files.json in build output directory

### Before opening, please confirm:

- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [x] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Build settings

### Is your feature request related to a problem? Please describe:

2025-05-12T09:20:31.373Z [INFO]: web:build: ▲ Next.js 14.2.9
2025-05-12T09:20:31.374Z [INFO]: web:build:
2025-05-12T09:20:31.470Z [INFO]: web:build: Creating an optimized production build ...
2025-05-12T09:23:22.511Z [INFO]: web:build: ✓ Compiled successfully
2025-05-12T09:23:22.516Z [INFO]: web:build: Linting and checking validity of types ...
2025-05-12T09:24:14.838Z [INFO]: Tasks: 13 successful, 13 total

Cached: 0 cached, 13 total
Time: 5m8.433s

2025-05-12T09:24:16.186Z [INFO]: # Completed phase: build
## Completed Frontend Build
2025-05-12T09:24:16.380Z [INFO]: ## Build completed successfully
2025-05-12T09:24:16.386Z [INFO]: # Starting caching...
2025-05-12T09:24:16.394Z [INFO]: # Creating cache artifact...
2025-05-12T09:25:56.532Z [INFO]: # Created cache artifact
2025-05-12T09:25:56.658Z [INFO]: # Uploading cache artifact...
2025-05-12T09:26:25.786Z [INFO]: # Uploaded cache artifact
2025-05-12T09:26:25.932Z [INFO]: # Caching completed
2025-05-12T09:26:25.947Z [ERROR]: !!! CustomerError: Can't find required-server-files.json in build output directory
2025-05-12T09:26:25.948Z [INFO]: # Starting environment caching...
2025-05-12T09:26:25.948Z [INFO]: # Environment caching completed

### Describe how you'd like this feature to work

always build failed. no build error. why?

Start with the provided Amplify build log and the reported Next.js 14.2.9 build settings, then reproduce the successful build followed by the missing required-server-files.json error. Done means identifying whether the failure is in the build configuration or Amplify's output detection and documenting a verified fix.

Written by the indexing model from the issue text.