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

This field is unused. Delete it.

codex-rs/core/src/turn_timing_tests.rs

@@ -102,7 +102,6 @@ fn response_item_records_turn_ttft_for_first_output_signals() {
         content: vec![ContentItem::OutputText {
             text: "hello".to_string(),
         }],
-        end_turn: None,
         phase: None,
     }));
 }
@@ -115,7 +114,6 @@ fn response_item_records_turn_ttft_ignores_empty_non_output_items() {
         content: vec![ContentItem::OutputText {
             text: String::new(),
         }],
-        end_turn: None,
         phase: None,
     }));
     assert!(!response_item_records_turn_ttft(