Function execute_chain 

pub async fn execute_chain(
    catalog: &Catalog,
    route_name: &str,
    legs: &[ChainLeg],
    req: &ChatRequest,
) -> Result<Completion, GatewayError>

Walk legs in order. Retryable failure (or open breaker) advances; the first non-retryable failure aborts. Returns AllLegsFailed if every leg fails.