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

This field is unused. Delete it.

codex-rs/core/src/session/mod.rs

@@ -2364,7 +2364,6 @@ impl Session {
             content: vec![ContentItem::InputText {
                 text: format!("Warning: {}", message.into()),
             }],
-            end_turn: None,
             phase: None,
         };