Support end_turn flag (#9698)

Experimental flag that signals the end of the turn.

codex-rs/core/tests/chat_completions_payload.rs

@@ -135,6 +135,7 @@ fn user_message(text: &str) -> ResponseItem {
         content: vec![ContentItem::InputText {
             text: text.to_string(),
         }],
+        end_turn: None,
     }
 }
 
@@ -145,6 +146,7 @@ fn assistant_message(text: &str) -> ResponseItem {
         content: vec![ContentItem::OutputText {
             text: text.to_string(),
         }],
+        end_turn: None,
     }
 }