1.43.0
(feat): Expose Stream[T].LastRetryMs() on the SSE runtime, mirroring LastEventID().
It returns the most recently advertised retry: reconnection time in
milliseconds (sticky per the SSE spec), so callers can read the directive
even when the server sends it in its own frame with no data: — in which
case the per-event StreamEvent.Retry is 0. The value persists across
reconnects.