librarian: deleting contents of generator-input directory

librarian seems to be removing files from generate-input directory:

```

2025/09/04 15:12:35 INFO cleaning directory path=/workspace/google-cloud-python
--
  | 2025/09/04 15:12:35 INFO removing file path=.librarian/generator-input/packages/google-cloud-language/.repo-metadata.json
```

It shouldn't be touching files there