Function evaluate_step 

pub fn evaluate_step(
    playbook: &Playbook,
    step_index: usize,
    alert_severity: &str,
    max_auto_severity: &str,
    require_approval: bool,
) -> StepExecutionResult

Evaluate a playbook step. Returns the result with approval gating. Steps that require approval and cannot be auto-approved will return StepStatus::PendingApproval without executing.