Post-rebase consolidation over the merged C7/C16/C10 work:
- _ensure_dm_conversation now records workspace ownership via
_remember_channel_team and bounds _dm_conversation_cache (cap 5000)
- module-level _slack_dm_cache (C7 standalone path) bounded oldest-first
- _user_is_bot_cache (C10) bounded with _trim_oldest_dict_entries
- caption-mode contract tests updated for the C7+C8 merged media path