Stop now aborts in-flight run/poll requests and finalizes the UI as cancelled immediately, preventing late chunks from rendering after cancellation. Add integration coverage for stop during active polling and before session initialization to lock in the behavior.

Trigger best-effort Docker container termination when cancelling an edit session and treat cancellation-related runtime failures as cancelled. This makes stop requests interrupt backend work sooner and keeps session state aligned with user-triggered cancellation.

Route stop-triggered container termination through the WorkspaceTooling facade to keep architecture boundaries intact while still hard-stopping agent runtimes. Also ignore late polling payloads after cancellation so no additional output renders once Stop is pressed.