Function execute_dag 

pub async fn execute_dag(
    workflow_name: &str,
    steps: &[DagWorkflowStep],
    input: &str,
    executor: Arc<dyn StepExecutor>,
) -> DagExecutionResult

Execute a DAG workflow using the provided step executor.

This is the core DAG execution engine. Steps with no dependencies (roots) run first. When a step completes, any step whose dependencies are now all satisfied is scheduled. Steps with no mutual dependencies run concurrently using tokio::task::JoinSet for true multi-threaded parallelism.