Body for POST /clusters/{cluster_id}/status.
target_cluster_id is required when to_status='merged' and ignored otherwise. The state-machine enforcement lives in GapClusterLifecycleService; this schema only constrains the wire shape.
Required when to_status == 'merged'; ignored otherwise.
Successful Response
Wire shape for a successful lifecycle mutation.
Mirrors ClusterTransitionResult from the lifecycle service with enum statuses flattened to their string values for SDK ergonomics.