Kafka gateway: Kafka Metadata API (key 3) — broker and topic discovery via Iggy

Part of #3560.

## Summary

Implement Kafka **Metadata (API key 3)** responses backed by Iggy stream/topic metadata instead of stub values.

## Motivation

Kafka clients bootstrap via Metadata before Produce/Fetch. Required for `kcat -L`, Java clients, and console tools.

## Depends on

- #3421
- #3533 — IggyBridge core

## Scope

- [ ] Handler for Metadata v0–v12 (per scoped version firewall in gateway)
- [ ] Return bridge broker id/host/port (advertised listener config)
- [ ] List topics from Iggy (or configured allowlist)
- [ ] Per-topic partition count aligned with Iggy topic partitions
- [ ] Golden-wire or integration tests for at least v0 and flexible (v9+) encodings

## Acceptance criteria

- [ ] `kcat -b localhost:9093 -L` lists broker and topics without error
- [ ] ApiVersions + Metadata handshake succeeds on same TCP connection

## References

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