The assistant message_end fan-out is fire-and-forget in the session layer
and can be parked on extension delivery while agent_end is flushed through
#endInFlight, so agent_end can overtake it. #finishPrompt then unsubscribes
the prompt turn and the mapAssistantMessageEnd fallback never runs: an ACP
client that only received agent_thought_chunk updates (thinking streamed,
text arrived only on the trailing message) stays stuck on the thinking
block with no visible answer. On agent_end, emit the last assistant
message's text before resolving the prompt when live-message progress shows
no text was ever delivered, and defer the live-state reset past that flush
so a late message_end cannot resurrect fresh progress and double-emit.
Fixes#4902
can1357
7c560c7151