fix: get responses API working again in Rust (#872)

I inadvertently regressed support for the Responses API when adding
support for the chat completions API in
https://github.com/openai/codex/pull/862. This should get both APIs
working again, but the chat completions codepath seems more complex than
necessary. I'll try to clean that up shortly, but I want to get things
working again ASAP.

codex-rs/core/src/client.rs

@@ -19,6 +19,7 @@ use tracing::debug;
 use tracing::trace;
 use tracing::warn;
 
+use crate::chat_completions::AggregateStreamExt;
 use crate::chat_completions::stream_chat_completions;
 use crate::client_common::Payload;
 use crate::client_common::Prompt;
@@ -111,7 +112,31 @@ impl ModelClient {
         match self.provider.wire_api {
             WireApi::Responses => self.stream_responses(prompt).await,
             WireApi::Chat => {
-                stream_chat_completions(prompt, &self.model, &self.client, &self.provider).await
+                // Create the raw streaming connection first.
+                let response_stream =
+                    stream_chat_completions(prompt, &self.model, &self.client, &self.provider)
+                        .await?;
+
+                // Wrap it with the aggregation adapter so callers see *only*
+                // the final assistant message per turn (matching the
+                // behaviour of the Responses API).
+                let mut aggregated = response_stream.aggregate();
+
+                // Bridge the aggregated stream back into a standard
+                // `ResponseStream` by forwarding events through a channel.
+                let (tx, rx) = mpsc::channel::<Result<ResponseEvent>>(16);
+
+                tokio::spawn(async move {
+                    use futures::StreamExt;
+                    while let Some(ev) = aggregated.next().await {
+                        // Exit early if receiver hung up.
+                        if tx.send(ev).await.is_err() {
+                            break;
+                        }
+                    }
+                });
+
+                Ok(ResponseStream { rx_event: rx })
             }
         }
     }

codex-rs/core/src/codex.rs

@@ -32,7 +32,6 @@ use tracing::trace;
 use tracing::warn;
 
 use crate::WireApi;
-use crate::chat_completions::AggregateStreamExt;
 use crate::client::ModelClient;
 use crate::client_common::Prompt;
 use crate::client_common::ResponseEvent;
@@ -864,7 +863,7 @@ async fn try_run_turn(
     sub_id: &str,
     prompt: &Prompt,
 ) -> CodexResult<Vec<ProcessedResponseItem>> {
-    let mut stream = sess.client.clone().stream(prompt).await?.aggregate();
+    let mut stream = sess.client.clone().stream(prompt).await?;
 
     // Buffer all the incoming messages from the stream first, then execute them.
     // If we execute a function call in the middle of handling the stream, it can time out.