Kafka gateway: Kafka Metadata API (key 3) — broker and topic discovery via Iggy #3534

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

• Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421
• Kafka gateway: Kafka gateway IggyBridge core — SDK client and stream/topic mapping #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: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)
• Tracking: feature - Order of Implementation discussion of kafka to iggy bridge #3253