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

This field is unused. Delete it.

codex-rs/protocol/src/protocol.rs

@@ -2867,7 +2867,6 @@ impl From<CompactedItem> for ResponseItem {
             content: vec![ContentItem::OutputText {
                 text: value.message,
             }],
-            end_turn: None,
             phase: None,
         }
     }