Support end_turn flag (#9698)

Experimental flag that signals the end of the turn.

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

@@ -193,6 +193,7 @@ impl Stream for AggregatedStream {
                             content: vec![ContentItem::OutputText {
                                 text: std::mem::take(&mut this.cumulative),
                             }],
+                            end_turn: None,
                         };
                         this.pending
                             .push_back(ResponseEvent::OutputItemDone(aggregated_message));