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/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -1616,6 +1616,7 @@
           "type": "object"
         },
         {
+          "description": "Display item emitted by the interruptible `clock.sleep` tool.",
           "properties": {
             "durationMs": {
               "format": "uint64",