fix: Emit pending items before Completed when stream closes gracefully

- When SSE stream closes with Ok(None), now emits pending assistant_item and reasoning_item BEFORE sending Completed event
- Previously would send Completed immediately without emitting accumulated messages, causing UI to hang with "working" state
- This fixes the hang when API returns 200 OK but SSE stream has no events
- Added debug logging for graceful stream closure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

llmx-rs/core/src/chat_completions.rs

@@ -645,7 +645,14 @@ async fn process_chat_sse<S>(
                 return;
             }
             Ok(None) => {
-                // Stream closed gracefully – emit Completed with dummy id.
+                // Stream closed gracefully – emit any pending items first, then Completed
+                debug!("Stream closed gracefully (Ok(None)), emitting pending items");
+                if let Some(item) = assistant_item.take() {
+                    let _ = tx_event.send(Ok(ResponseEvent::OutputItemDone(item))).await;
+                }
+                if let Some(item) = reasoning_item.take() {
+                    let _ = tx_event.send(Ok(ResponseEvent::OutputItemDone(item))).await;
+                }
                 let _ = tx_event
                     .send(Ok(ResponseEvent::Completed {
                         response_id: String::new(),