tests/sync(feat[test_git]): Cover all 13 update_repo error steps

why: 7 of 13 error steps in GitSync.update_repo() had no test coverage.
what:
- Add test_update_repo_obtain_failure_recorded
- Add test_update_repo_set_remotes_failure_recorded
- Add test_update_repo_remote_name_failure_recorded
- Add test_update_repo_status_failure_recorded
- Add test_update_repo_stash_save_failure_recorded
- Add test_update_repo_rebase_invalid_upstream_recorded
- Add test_update_repo_rebase_conflict_recorded
- Add test_update_repo_stash_pop_failure_recorded
- Add test_sync_result_multiple_errors for multi-error accumulation

Author: tony