fix(codex-api): handle Chat Completions DONE sentinel (#8708)

Context
- This code parses Server-Sent Events (SSE) from the legacy Chat
Completions streaming API (wire_api = "chat").
- The upstream protocol terminates a stream with a final sentinel event:
data: [DONE].
- Some of our test stubs/helpers historically end the stream with data:
DONE (no brackets).

How this was found
- GitHub Actions on Windows failed in codex-app-server integration tests
with wiremock verification errors (expected multiple POSTs, got 1).

Diagnosis
- The job logs included: codex_api::sse::chat: Failed to parse
ChatCompletions SSE event ... data: DONE.
- eventsource_stream surfaces the sentinel as a normal SSE event; it
does not automatically close the stream.
- The parser previously attempted to JSON-decode every data: payload.
The sentinel is not JSON, so we logged and skipped it, then continued
polling.
- On servers that keep the HTTP connection open after emitting the
sentinel (notably wiremock on Windows), skipping the sentinel meant we
never emitted ResponseEvent::Completed.
- Higher layers wait for completion before progressing (emitting
approval requests and issuing follow-up model calls), so the test never
reached the subsequent requests and wiremock panicked when its
expected-call count was not met.

Fix
- Treat both data: [DONE] and data: DONE as explicit end-of-stream
sentinels.
- When a sentinel is seen, flush any pending assistant/reasoning items
and emit ResponseEvent::Completed once.

Tests
- Add a regression unit test asserting we complete on the sentinel even
if the underlying connection is not closed.

codex-rs/codex-api/src/sse/chat.rs

@@ -30,6 +30,21 @@ pub(crate) fn spawn_chat_stream(
     ResponseStream { rx_event }
 }
 
+/// Processes Server-Sent Events from the legacy Chat Completions streaming API.
+///
+/// The upstream protocol terminates a streaming response with a final sentinel event
+/// (`data: [DONE]`). Historically, some of our test stubs have emitted `data: DONE`
+/// (without brackets) instead.
+///
+/// `eventsource_stream` delivers these sentinels as regular events rather than signaling
+/// end-of-stream. If we try to parse them as JSON, we log and skip them, then keep
+/// polling for more events.
+///
+/// On servers that keep the HTTP connection open after emitting the sentinel (notably
+/// wiremock on Windows), skipping the sentinel means we never emit `ResponseEvent::Completed`.
+/// Higher-level workflows/tests that wait for completion before issuing subsequent model
+/// calls will then stall, which shows up as "expected N requests, got 1" verification
+/// failures in the mock server.
 pub async fn process_chat_sse<S>(
     stream: S,
     tx_event: mpsc::Sender<Result<ResponseEvent, ApiError>>,
@@ -57,6 +72,31 @@ pub async fn process_chat_sse<S>(
     let mut reasoning_item: Option<ResponseItem> = None;
     let mut completed_sent = false;
 
+    async fn flush_and_complete(
+        tx_event: &mpsc::Sender<Result<ResponseEvent, ApiError>>,
+        reasoning_item: &mut Option<ResponseItem>,
+        assistant_item: &mut Option<ResponseItem>,
+    ) {
+        if let Some(reasoning) = reasoning_item.take() {
+            let _ = tx_event
+                .send(Ok(ResponseEvent::OutputItemDone(reasoning)))
+                .await;
+        }
+
+        if let Some(assistant) = assistant_item.take() {
+            let _ = tx_event
+                .send(Ok(ResponseEvent::OutputItemDone(assistant)))
+                .await;
+        }
+
+        let _ = tx_event
+            .send(Ok(ResponseEvent::Completed {
+                response_id: String::new(),
+                token_usage: None,
+            }))
+            .await;
+    }
+
     loop {
         let start = Instant::now();
         let response = timeout(idle_timeout, stream.next()).await;
@@ -70,24 +110,8 @@ pub async fn process_chat_sse<S>(
                 return;
             }
             Ok(None) => {
-                if let Some(reasoning) = reasoning_item {
-                    let _ = tx_event
-                        .send(Ok(ResponseEvent::OutputItemDone(reasoning)))
-                        .await;
-                }
-
-                if let Some(assistant) = assistant_item {
-                    let _ = tx_event
-                        .send(Ok(ResponseEvent::OutputItemDone(assistant)))
-                        .await;
-                }
                 if !completed_sent {
-                    let _ = tx_event
-                        .send(Ok(ResponseEvent::Completed {
-                            response_id: String::new(),
-                            token_usage: None,
-                        }))
-                        .await;
+                    flush_and_complete(&tx_event, &mut reasoning_item, &mut assistant_item).await;
                 }
                 return;
             }
@@ -101,16 +125,25 @@ pub async fn process_chat_sse<S>(
 
         trace!("SSE event: {}", sse.data);
 
-        if sse.data.trim().is_empty() {
+        let data = sse.data.trim();
+
+        if data.is_empty() {
             continue;
         }
 
-        let value: serde_json::Value = match serde_json::from_str(&sse.data) {
+        if data == "[DONE]" || data == "DONE" {
+            if !completed_sent {
+                flush_and_complete(&tx_event, &mut reasoning_item, &mut assistant_item).await;
+            }
+            return;
+        }
+
+        let value: serde_json::Value = match serde_json::from_str(data) {
             Ok(val) => val,
             Err(err) => {
                 debug!(
                     "Failed to parse ChatCompletions SSE event: {err}, data: {}",
-                    &sse.data
+                    data
                 );
                 continue;
             }
@@ -362,6 +395,16 @@ mod tests {
         body
     }
 
+    /// Regression test: the stream should complete when we see a `[DONE]` sentinel.
+    ///
+    /// This is important for tests/mocks that don't immediately close the underlying
+    /// connection after emitting the sentinel.
+    #[tokio::test]
+    async fn completes_on_done_sentinel_without_json() {
+        let events = collect_events("event: message\ndata: [DONE]\n\n").await;
+        assert_matches!(&events[..], [ResponseEvent::Completed { .. }]);
+    }
+
     async fn collect_events(body: &str) -> Vec<ResponseEvent> {
         let reader = ReaderStream::new(std::io::Cursor::new(body.to_string()))
             .map_err(|err| codex_client::TransportError::Network(err.to_string()));