Support end_turn flag (#9698)

Experimental flag that signals the end of the turn.

codex-rs/core/src/thread_manager.rs

@@ -411,6 +411,7 @@ mod tests {
             content: vec![ContentItem::OutputText {
                 text: text.to_string(),
             }],
+            end_turn: None,
         }
     }
     fn assistant_msg(text: &str) -> ResponseItem {
@@ -420,6 +421,7 @@ mod tests {
             content: vec![ContentItem::OutputText {
                 text: text.to_string(),
             }],
+            end_turn: None,
         }
     }