Related to #613
When all PBS datastore queries fail (e.g., due to network issues or PBS
downtime), the system was clearing all backups and showing an empty list.
This adds the same preservation logic that exists for PVE storage backups.
Changes:
- Add shouldPreservePBSBackups() helper function
- Track datastore query success/failure counts in pollPBSBackups()
- Preserve existing backups when all datastore queries fail
- Add comprehensive unit tests for PBS backup preservation logic
This ensures users can still see their backup history even during
temporary connectivity issues with PBS, matching the behavior already
implemented for PVE storage backups.
