Add Cluster Mode Enabled (CME) testing support

[detemmienation]

This PR implements CME (Cluster Mode Enabled) testing support, addressing issue #3.

What's added

Three classes in src/valkey_test_case.py:

• ClusterInfo — exposes typed accessors over a CLUSTER INFO reply (is_cluster_ok, cluster_known_nodes, cluster_slots_assigned).
• ClusterNodeHandle — a single cluster-mode node; wraps the standard CLUSTER commands (MEET / REPLICATE / ADDSLOTSRANGE) and the waiters for the cluster to converge.
• ClusterTestCase — orchestrates bootstrap. setup_cluster(num_shards, num_replicas_per_shard) starts the nodes, gossips them together, evenly distributes the 16384 hash slots across primaries, optionally attaches replicas, waits for cluster_state:ok, and returns a topology-awareValkeyCluster client.

Cluster formation is done purely through standard CLUSTER commands, and it reuses the framework's existing port reservation, server lifecycle, and nodes.conf cleanup.

Tests

tests/test_cluster_mode.py:

• 3-shard cluster: healthy state, all slots assigned, read/write through the cluster client.
• Slot distribution (2/3/5 shards): CLUSTER SLOTS covers 0..16383 exactly once, no overlap.
• Replica topologies (3-shard + 1-replica, 2-shard + 2-replica): attachment, online counts, read/write.