Function execute_streaming 

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

Streaming executor: for each leg, start the stream and peek the first item. First leg to yield an item is committed; failures before the first item fall back. After commitment there is no fallback (caller surfaces mid-stream errors as SSE error events).

Delegates to execute_streaming_with_timeouts with StreamTimeouts::default.