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

This field is unused. Delete it.

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -2503,7 +2503,6 @@ async fn resume_agent_restores_closed_agent_and_accepts_send_input() {
                 content: vec![ContentItem::InputText {
                     text: "materialized".to_string(),
                 }],
-                end_turn: None,
                 phase: None,
             })]),
             AuthManager::from_auth_for_testing(CodexAuth::from_api_key("dummy")),