Function apply_task_event 

pub fn apply_task_event(
    dag: &TaskDag,
    run: &TaskRun,
    event: TaskEvent,
) -> Result<TaskRun, TaskEventError>

Apply a state event and return the next run projection.

This reducer is deliberately conservative: only ready tasks can start, only running tasks can pass/fail, terminal tasks cannot be changed, and newly unblocked pending tasks become ready.