Fix async producer transaction behavior + add transactional produce benchmark test

[fangnx]

What

Key Changes

AIOProducer Transaction Fixes

• Fixed commit_transaction(): add call await self.flush() before committing to ensure both Python-layer buffer and librdkafka queue are flushed. Previously the Python-layer batch_processor buffer was not flushed
• Fixed abort_transaction(): similarly, add call await self.purge() to cancel pending futures and clear Python-layer buffer

Producer Strategy Transaction Support

• Update to support produce with transaction

Transaction Performance Testing

• Added test_basic_produce_with_transaction benchmark test (per-message transaction) for sync vs async
• Created transaction-specific performance bounds in transaction_benchmark_bounds.json

File Organization

• Renamed benchmark_metrics.py → producer_benchmark_metrics.py, benchmark_bounds.json → producer_benchmark_bounds.json for consistency
• Clean up ducktape README

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA:

Test & Review

Open questions / Follow-ups

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