Support end_turn flag (#9698)

Experimental flag that signals the end of the turn.

codex-rs/app-server/tests/suite/list_resume.rs

@@ -307,6 +307,7 @@ async fn test_list_and_resume_conversations() -> Result<()> {
         content: vec![ContentItem::InputText {
             text: fork_history_text.to_string(),
         }],
+        end_turn: None,
     }];
     let resume_with_history_req_id = mcp
         .send_resume_conversation_request(ResumeConversationParams {

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -218,6 +218,7 @@ async fn thread_resume_supports_history_and_overrides() -> Result<()> {
         content: vec![ContentItem::InputText {
             text: history_text.to_string(),
         }],
+        end_turn: None,
     }];
 
     // Resume with explicit history and override the model.