Kafka gateway: Kafka gateway Phase 2 — E2E tests with consumer groups

Part of #3560.

## Summary

End-to-end validation of Phase 2 consumer group support.

## Depends on

- #3542 — OffsetCommit / OffsetFetch
- #3543 — LeaveGroup / rebalance

## Scope

- [ ] `kafka-console-consumer.sh --bootstrap-server ... --group test-group --topic ...`
- [ ] Multi-consumer same group integration test
- [ ] Offset commit + bridge restart + resume test
- [ ] Update README: Phase 2 capabilities and limits
- [ ] Update discussion #3253 with issue links

## Acceptance criteria

- [ ] All Phase 2 success metrics from discussion #3253 met
- [ ] CI covers consumer group happy path

## References

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

Read the rollout plan in discussion #3253 and the dependent issues #3542 and #3543 first; use the `kafka-console-consumer.sh` command as the starting entry point for the E2E checks. Done means the listed multi-consumer, offset-resume, and CI happy-path checks pass, with the README and discussion updated as scoped.

Written by the indexing model from the issue text.