java: resolve generation diff

[zhumin8]

java-iam-policy/.repo-metadata.json is changed in googleapis/google-cloud-java#13378, with no generation_config.yaml or librarian.yaml update.
This is causing generation diff: https://github.com/googleapis/google-cloud-java/actions/runs/27148297428/job/80132085152
Verify this is intended, and update config files accordingly.