Function validate_workflow_graph 

pub fn validate_workflow_graph(
    executors: &HashMap<String, Arc<dyn Executor>>,
    edge_groups: &[EdgeGroup],
    start: &str,
    output_executors: &[String],
    intermediate_executors: &[String],
) -> Result<(), WorkflowValidationError>

Validate a workflow graph. Rust analogue of validate_workflow_graph.

Checks, in order:

1. the start executor is registered;
2. every edge references a registered executor;
3. no directed (source, target) edge is duplicated;
4. every executor is reachable from the start node;
5. the workflow-output designation (output_executors/ intermediate_executors) does not overlap and only names known executors.