Function spawn_chained_tasks 

pub async fn spawn_chained_tasks(
    env: &StreamEnv,
    job_state: Arc<dyn StateStore>,
    state_work_dir: Option<&Path>,
    job_runtime: &ChainJobRuntime,
    ack_tx: &Sender<ControlMsg>,
    error_tx: &Sender<StreamError>,
    chain_id: u64,
    task_id_counter: &mut u32,
) -> StreamResult<Vec<PendingTask>>

Build one pending chain task per subtask from the env’s ChainJob.