Coordinator: convert join_group to async def

[dpkp]

• 1. Add _process_join_group_response(response) and _process_sync_group_response(response) -> bytes (raise-or-return). Test against existing parametrized response cases.
• 2. Add _on_join_prepare_async / _on_join_complete_async in base; override in ConsumerCoordinator.
• 3. Add _do_join_and_sync_async. MockBroker test: script JoinGroup → SyncGroup, assert returned bytes; one test per error code, assert correct exception type.
• 4. Add join_group_async and ensure_active_group_async.
• 5. Replace join_group and ensure_active_group bodies with manager.run(...) facades.
• 6. Delete dead code
• AsyncConsumerRebalanceListener
• fixup
• 8. Wrap listener call site with a time.monotonic() measurement and log.warning if it exceeds slow_task_threshold_secs
• 9. few more tests
• Drop unused join_group()