fix: manual preservation of Version.java files

[diegomarquezp]

Description

This PR addresses an issue where Version.java files were either incorrectly preserved or not generated during a library's initial generation due to an overly relaxed deep-preserve-regex rule in the OwlBot configuration templates.

To fix this edge case, we are moving the responsibility of preserving existing Version.java files away from the OwlBot YAML regex and directly into our hermetic build scripts.

Implementation Details

1. Removed Obsolete Regex: Removed the deep-preserve-regex rule targeting Version.java from owlbot.yaml.monorepo.j2.
2. Conditional Backup/Restore: Extracted new backup and restore utilities into utilities.sh and invoked them in postprocess_library.sh. The script now dynamically scans for existing Version.java files, backs them up to a temporary directory before executing owl-bot copy-code, and restores them immediately after.

Why this approach?

• First Generation: Since there are no existing Version.java files, the script does nothing, allowing OwlBot to generate and retain the new file as intended.
• Release Please Interoperability: Release Please regularly updates the version string inside existing Version.java files. Because our backup_version_java utility copies the existing file out of the repository before owl-bot copy-code executes, the file containing the updated Release Please version string is safely stored in mktemp -d. After owl-bot copy-code writes the newly generated, generic Version.java file to the repo, restore_version_java overwrites it with our backup. This guarantees the Release Please version updates are perfectly preserved.
• Hermetic Safety: The backups are stored in mktemp -d which guarantees they are unaffected by OwlBot's deep-remove-regex folder wipes.

Testing

• Added unit tests
• Once approach is validated: modify the chore/test-hermetic-build branch with arbitrary values in Version.java files and confirm they remain untouched via integration test

[gemini-code-assist]

Code Review

This pull request introduces a workaround to prevent owl-bot copy-code from omitting Version.java files by implementing a backup and restore mechanism during the post-processing phase. The changes include new utility functions backup_version_java and restore_version_java in utilities.sh, and their integration into postprocess_library.sh. Review feedback highlights potential issues with shell word splitting in the new functions and suggests using find -print0 with a while read loop to safely handle filenames that may contain spaces.

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[blakeli0]

cc: @zhumin8 since this may affect librarian migration.