Function cleanup_throwaway_block 

fn cleanup_throwaway_block(
    mstore: &Arc<Store>,
    broker: &Broker,
    throwaway_block_id: &str,
)

Kill the throwaway controller, delete its block row, and purge any MPS- persisted history under its scope — mirrors COMMAND_DELETE_SUB_BLOCK’s kill-then-delete ordering (server/websocket.rs). Best-effort: this runs after the turn is already over, so a failure here is a leftover row/controller to log, not a failed /btw answer.