Function chat_stream_signalled 

pub async fn chat_stream_signalled(
    target: &TargetEndpoint,
    body: &Value,
    timeout: Duration,
    first_token: Option<Sender<Instant>>,
) -> Result<ChatOutcome>

2026-10-04: chat_stream, plus a signal: first_token, when given, receives the instant the first token-carrying chunk arrived (the instant ttft_ms is measured to) while the stream is still running. A request that ends before any token drops the sender, so the receiver sees an error instead of waiting.