Forward Rate limits to the UI (#3965)

We currently get information about rate limits in the response headers.
We want to forward them to the clients to have better transparency.
UI/UX plans have been discussed and this information is needed.

codex-rs/core/src/chat_completions.rs

@@ -716,6 +716,9 @@ where
                     // Not an assistant message – forward immediately.
                     return Poll::Ready(Some(Ok(ResponseEvent::OutputItemDone(item))));
                 }
+                Poll::Ready(Some(Ok(ResponseEvent::RateLimits(snapshot)))) => {
+                    return Poll::Ready(Some(Ok(ResponseEvent::RateLimits(snapshot))));
+                }
                 Poll::Ready(Some(Ok(ResponseEvent::Completed {
                     response_id,
                     token_usage,