Support end_turn flag (#9698)

Experimental flag that signals the end of the turn.

codex-rs/protocol/src/protocol.rs

@@ -1607,6 +1607,7 @@ impl From<CompactedItem> for ResponseItem {
             content: vec![ContentItem::OutputText {
                 text: value.message,
             }],
+            end_turn: None,
         }
     }
 }