Delete unused ResponseItem::Message.end_turn (#19605)

This field is unused. Delete it.

codex-rs/state/src/extract.rs

@@ -177,7 +177,6 @@ mod tests {
             content: vec![ContentItem::InputText {
                 text: "hello from response item".to_string(),
             }],
-            end_turn: None,
             phase: None,
         });