## What changed
- Add an optional `delivery` field to agent message events and app-server
`agentMessage` items, with `"async"` identifying a user-visible message sent
without ending the current turn.
- Preserve the marker through legacy event conversion, thread history
materialization, replay, and generated JSON and TypeScript schemas.
- Require delivery of async agent message completion notifications on the
in-process app-server transport.
## Testing
- Cover preservation of async delivery metadata in thread history.
- Cover delivery-required classification for async agent messages.
GitOrigin-RevId: f92ba25de1293ee271404badfdf0d117d6530329