test(cluster): cover the VM membership and RAFT recovery (4/7)

[marceloneppel]

Issue

The membership and RAFT recovery slices need their unit coverage (the code slice precedes its tests, mirroring the logical-replication stack's 6/8 pattern).

Solution

• tests/unit/test_cluster_membership.py: the defer-path guard (a deferred re-add never drops or duplicates members_ips), departed early-exits, reconfigure/add-cluster-member flows, update_member_ip IP-change handling, and the RAFT recovery transitions — ports of the VM charm's test cases.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.