Move sleep items to the extension-owned lifecycle path (#33159)

## What changed

- Define `SleepItem` in `codex-extension-items` and carry `clock.sleep` events through `TurnItem::Extension`.
- Preserve sleep persistence, thread-history reconstruction, and the app-server's `type: "sleep"` schema while reusing the extension-owned type.

## Testing

- Add serialization coverage for the `clock.sleep` extension wire shape and its protocol wrapper.
- Update sleep lifecycle and thread-history tests to exercise the extension-owned item.

GitOrigin-RevId: 34bb33ed9b90a09bf4744aafdb56e019c717a168

codex-rs/ext/items/src/tests.rs

@@ -3,6 +3,7 @@ use serde_json::json;
 
 use super::ExtensionItem;
 use super::image_generation::ImageGenerationItem;
+use super::sleep::SleepItem;
 use super::web_search::WebSearchAction;
 use super::web_search::WebSearchItem;
 
@@ -84,6 +85,28 @@ fn web_search_item_preserves_stable_wire_shape() {
     );
 }
 
+#[test]
+fn sleep_item_preserves_stable_wire_shape() {
+    let item = ExtensionItem::Sleep(SleepItem {
+        id: "sleep-1".to_string(),
+        duration_ms: 1_000,
+    });
+    let value = serde_json::to_value(&item).expect("serialize extension item");
+
+    assert_eq!(
+        value,
+        json!({
+            "kind": "clock.sleep",
+            "id": "sleep-1",
+            "durationMs": 1_000,
+        })
+    );
+    assert_eq!(
+        serde_json::from_value::<ExtensionItem>(value).expect("deserialize extension item"),
+        item
+    );
+}
+
 #[test]
 fn unknown_extension_kind_is_rejected() {
     let value = json!({