Follow-up to HuangYuChuh's #17384 cherry-pick:
- Use defensive getattr+logger.debug for delete_message lookup, mirroring
the sibling _try_send_fresh_final cleanup pattern at L820+. Platforms
that don't implement delete_message no longer raise AttributeError; the
failure path now logs at debug for diagnosability instead of silently
swallowing.
- Add three regression tests in tests/gateway/test_stream_consumer.py:
- delete_message awaited on happy-path exit with stale id
- delete_message NOT awaited when no fallback chunks reached the user
- no crash on adapters that lack delete_message (spec-restricted mock)