Stamp conversation history items with creation times (#38272)

## What changed

- Add fractional Unix creation times to locally authored user, developer, agent, and tool-output items when they enter durable conversation history.
- Preserve any creation time already supplied with an item across subsequent requests, remote compaction, and resumed history.

## Testing

- Cover timestamp assignment and preservation in session, client, tool, agent-spawn, and remote-compaction tests.

GitOrigin-RevId: 431d1770b9746005c808bc0a43953845166d68ad

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

@@ -382,8 +382,15 @@ mod tests {
         session
             .record_conversation_items(&turn, std::slice::from_ref(&history_item))
             .await;
-        let mut expected_history_item = history_item.clone();
-        expected_history_item.set_turn_id_if_missing(&turn_id);
+        let expected_history_item = strip_response_item_id(
+            session
+                .clone_history()
+                .await
+                .raw_items()
+                .next()
+                .expect("history item")
+                .clone(),
+        );
         let raw_history_event = rx.recv().await.expect("history raw response item event");
         let EventMsg::RawResponseItem(raw_history_item) = raw_history_event.msg else {
             panic!("expected raw response item event");

codex-rs/core/src/tools/router_tests.rs

@@ -545,8 +545,13 @@ async fn extension_tool_executors_are_model_visible_and_dispatchable() -> anyhow
     session
         .record_conversation_items(&turn, std::slice::from_ref(&history_item))
         .await;
-    let mut expected_history_item = history_item.clone();
-    expected_history_item.set_turn_id_if_missing(&turn.sub_id);
+    let expected_history_item = session
+        .clone_history()
+        .await
+        .raw_items()
+        .next()
+        .expect("history item")
+        .clone();
 
     let router = test_tool_router(
         step_context.as_ref(),