Stop an AI Assistant chat turn

Ask the turn whose user message sits at sequence (the streaming route's turn frame names it) to stop at its next check. The turn stops before its model call, or within a second while the model writes, and lands as a result whose reply carries failed and reads Stopped by you; the workspace's allowance is not charged for it. Idempotent, and harmless once the turn has landed: a Stop nobody is listening for expires on its own.