Kafka gateway: Kafka DescribeGroups and ListGroups admin APIs

Part of #3560.

## Summary

Implement **DescribeGroups (15)** and **ListGroups (16)** for observability and `kafka-consumer-groups.sh` compatibility.

## Depends on

- #3541 — consumer group state exists
- #3539 — Phase 1

## Scope

- [ ] ListGroups: active groups on coordinator
- [ ] DescribeGroups: members, assignment, state (Stable, CompletingRebalance, etc.)
- [ ] Integration test with running consumer group

## Acceptance criteria

- [ ] Admin can inspect group membership via Kafka CLI tools

## References

- Rollout plan: https://github.com/apache/iggy/discussions/3253#discussioncomment-17181030

Start by reviewing issues #3541 and #3539, then locate the Kafka gateway's existing admin API handling. Run the relevant gateway tests and inspect how consumer group state, members, and assignments are represented. Done means ListGroups and DescribeGroups work for a running consumer group and are usable through kafka-consumer-groups.sh, with an integration test covering the behavior.

Written by the indexing model from the issue text.