testcluster: fix CrashNode isolation using Partitioner

[pav-kv]

CrashNode's circuit breaker isolation is insufficient: it only blocks outbound RPCs from the crashing node. Server-side responses on existing gRPC streams (e.g. MsgAppResp sent during raft snapshot application) can still escape after CrashClone, leaking false durability signals into the cluster.

This PR replaces circuit breakers with the Partitioner's bidirectional stream interceptors, which block both SendMsg and RecvMsg on client streams.

Commit 1 moves the Partitioner from kvnemesis into TestCluster:

• New EnablePartitioner flag on TestClusterArgs
• TestCluster handles interceptor registration (AddServer) and address mapping (Start, startServer)
• kvnemesis no longer manages the Partitioner directly; uses tc.Partitioner()

Commit 2 fixes CrashNode isolation:

• Replaces isolateNodeFromPeers (circuit breakers) with bidirectional partitions via AddPartition/RemovePartition
• Partitions are added before CrashClone and removed after stopServerLocked

Fixes #166145