run the following command:

go run ./cmd/librarian/ generate -push

It fails with the following error:

2025/09/17 13:18:56 failed to create pull request: POST https://api.github.com/repos/googleapis/librarian/pulls: 422 Validation Failed [{Resource:PullRequest Field:base Code:missing_field Message:}]

full log: https://paste.googleplex.com/4814140543729664