GeezerVison

A master with no replicas is a valid Redis Cluster topology. When a long-lived client receives MOVED to a master absent from its map, cluster_update_slot currently throws an incomplete-master exception if the previous owner has no replica table. This prevents recovery after adding a master or after consecutive failovers.

Only search for a promoted replica when a replica table exists. Otherwise, use the existing new-node redirect path. Known-replica promotion still remaps the keyspace and preserves remap errors.

The per-slot path is the one develop already takes for an unknown MOVED target, so no extra keyspace remap is introduced. Remapping during a failover still uses only the configured seeds; that is unchanged and out of scope here.

The regression supplies a stale slot map through a one-shot seed and exercises real MOVED replies, repeated reads, and a subsequent write against the test cluster. It does not alter cluster ownership or require stopping replicas in CI.

Closes #2740.
Closes #2828.

Validation

• The regression fails on unchanged develop and passes with the patch, without authentication, with password authentication, and with the ACL username/password configuration used in CI.
• Real Linux slot migration from a three-master cluster to a newly added master reproduces cluster_update_slot failing if new master added #2740 on develop; the existing client recovers with the patch.
• A real A → B → A takeover sequence reproduces RedisClusterException: Redis Cluster's master data is incomplete. Cluster is in invalid state. #2828 on develop; the existing client recovers after both failovers with the patch. Fresh clients succeed in both baseline scenarios.
• The updated regression also passes from a generated PECL tarball, which includes its fixture.
• Focused checks cover persistent connections, slot caching, prefix/serializer options, writes as the first redirected operation, and recovery after an unreachable MOVED target. Valgrind reports no memory errors or definitely, indirectly, or possibly lost allocations on the tested new-node paths.