Kafka gateway: Kafka CreateTopics API (key 19) — Iggy stream/topic provisioning #3538

Part of #3560.

Summary

Implement Kafka CreateTopics (API key 19) admin handler that creates Iggy streams and topics.

Motivation

Allows kafka-topics.sh --create and auto-topic creation from producers against the gateway.

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 (ensure_stream_and_topic)

Scope

• Decode CreateTopics request (scoped versions v2–v7)
• Map num_partitions, replication_factor (document: RF=1 or ignored; Iggy replication model)
• Create Iggy stream/topic via SDK; idempotent if exists
• Per-topic error in response (TOPIC_ALREADY_EXISTS, etc.)
• Config validation: reject unsupported Kafka topic configs with documented limits

Acceptance criteria

• kafka-topics.sh --create --topic test --bootstrap-server localhost:9093 succeeds
• Created topic appears in Metadata and accepts Produce

References

• Rollout plan: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)