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/tui/src/thread_transcript.rs

@@ -230,7 +230,7 @@ fn fallback_transcript_cell(item: &ThreadItem) -> Option<PlainHistoryCell> {
         | ThreadItem::AgentMessage { .. }
         | ThreadItem::Plan { .. }
         | ThreadItem::Reasoning { .. }
-        | ThreadItem::Sleep { .. } => return None,
+        | ThreadItem::Sleep(_) => return None,
     };
     (!lines.is_empty()).then(|| PlainHistoryCell::new(lines))
 }