Add multi-shard Raft coordination tests

[osvaldoandrade]

Test Raft consensus coordination across shards.

Current gap:

• Raft tests exist for single-shard
• Missing: multi-shard Raft group coordination, mux transport, per-shard FSM

Goal:

• Test multi-shard Raft setup
• Test mux transport (sharing one port for multiple Raft groups)
• Test per-shard FSM isolation

Implementation:

• Create internal/raft/multi_shard_test.go
• Simulate 3-node cluster with 2 shards each
• Test leader election per shard
• Test mux transport routing

Acceptance criteria:

• Multi-shard scenarios covered
• Mux transport tested
• FSM isolation verified

Part of: #654
Requires: #645 (foundations), testutil/wait