Kafka gateway: Kafka transactional APIs — UNSUPPORTED_VERSION stub and idempotence docs #3545

Part of #3560.

Summary

Phase 3: handle transactional producer APIs with explicit unsupported responses until Iggy supports transactional writes.

Motivation

Iggy does not support transactional writes today. Workaround: return UNSUPPORTED_VERSION (35) for InitProducerId (22), AddPartitionsToTxn (24), AddOffsetsToTxn (25), EndTxn (26), TxnOffsetCommit (28).

Document that Kafka 3.x idempotent producers fall back to at-least-once.

Depends on

• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1 (minimum)

Scope

• Handlers return UNSUPPORTED_VERSION or documented stub behavior
• README section: transactions not supported; idempotent producer behavior
• Tests assert error codes, no panic

References

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