Tests /help, /status, /new, /stop, /commands through the full adapter
background-task pipeline. Validates command dispatch, session lifecycle,
and response delivery without any LLM involvement.
Fixtures and helpers for driving messages through the full async
pipeline: adapter.handle_message → background task → GatewayRunner
command dispatch → adapter.send (mocked).
Uses the established _make_runner pattern (object.__new__) to skip
filesystem side effects while exercising real command dispatch logic.