Get operation

Administrative execution only. Retries keep identity. Intentional terminal rerun has a new ID and previous_operation_id. Cancellation does not promise universal rollback; already-terminal state and racing completion may win. projection_rebuild, retrieval_configuration and backfill Operations require corpus_id; when succeeded they require result naming the activated logical generation, or for a backfill the generation it filled. A backfill also carries backfill, and its counters versions_in_scope, versions_done, versions_skipped (with skipped_<reason>) and segments. A quarantine_reprocess carries corpus_id and quarantine_reprocess, and its counters versions_in_scope, versions_recovered, versions_quarantined and versions_skipped (with skipped_<reason>); it has no result. Only a backfill and a quarantine_reprocess can be paused. vector_space_promotion, plugin_activation and plugin_rollback carry admin, with no corpus_id. Their counters records_scanned, versions_missing and segments_missing describe the preflight; required_missing excludes gaps retained from an earlier rollback. Success publishes routing; rollback recovery continues in the background after success.