librarian(bug): Librarian pull request titles should follow conventional commit spec using prefix `feat`

In https://github.com/googleapis/google-cloud-python/pull/14420, the title was `Librarian generate pull request: 20250911T204709Z`. Following the conventional commit spec, we should use `feat: generate libraries 20250911T204709Z` or something similar, as long as we're using the conventional commit prefix `feat`