test: add integration tests for rebase progress, status, cleanup, and summary

Add 6 new integration tests for Phase 3 rebase features:
- rebase_progress_reporting: verifies [1/3], [2/3], [3/3] progress output
- rebase_status_no_state: verifies "No chain rebase in progress" message
- rebase_status_during_conflict: verifies status shows Conflict/Pending
- rebase_cleanup_backups: verifies --cleanup-backups deletes backup branches
- rebase_no_cleanup_without_flag: verifies backups preserved without flag
- rebase_summary_report_with_skipped_branches: verifies summary counts

Also improve assertion messages in rebase_subcommand_simple for better
debugging on failure.

Author: dashed