Kafka gateway: Kafka LeaveGroup and consumer group rebalance semantics

Part of #3560.

## Summary

Implement **LeaveGroup (13)** and complete rebalance edge cases for consumer groups.

## Depends on

- #3541 — JoinGroup / Heartbeat / SyncGroup

## Scope

- [ ] Graceful LeaveGroup removes member and triggers rebalance
- [ ] Static membership / generation id behavior documented
- [ ] Consumer failure without LeaveGroup handled via session timeout (#3541)
- [ ] Tests: N consumers join/leave, partitions reassigned

## Acceptance criteria

- [ ] Rebalance on heartbeat timeout verified in test
- [ ] Graceful shutdown releases partitions promptly

Begin with issue #3541 to understand the JoinGroup, Heartbeat, and SyncGroup dependency, then trace the Kafka gateway's consumer-group handling for LeaveGroup and timeout paths. Add tests covering N consumers joining and leaving, heartbeat-timeout rebalances, partition reassignment, and prompt release during graceful shutdown; done means all listed acceptance criteria pass.

Written by the indexing model from the issue text.