Add enqueueMany method and fix flaky tests

- Add enqueueMany method to efficiently send multiple messages
- Fix flaky tests by removing dependency on message order:
  - Create separate arrays for tracking messages from each listener
  - Add allMessages array to track all received messages
  - Update assertions to check for message inclusion rather than exact order
  - Use message presence for waitFor conditions instead of array length

Co-Authored-By: Claude <noreply@anthropic.com>