Stop an agent prompt revision that is being streamed

Ask the revision streaming under edit_id (its first frame) to stop. Cooperative: the revision reads the request once a second while the model writes and again before the prompt is handed back, and ends there, its trail on the last step with Stopped by you on the one it interrupted and its stream in an error frame with reason stopped. A revision that has already landed is unaffected. The turn a stopped revision claimed is given back.