## What changed
- Add an optional `initialItems` field to `thread/realtime/start` for seeding
realtime V3 session history with `user`, `developer`, and `assistant` text.
- Encode the items in Frameless Bidi session bootstrap payloads for websocket
and WebRTC transports while preserving existing payloads when the list is
omitted or empty.
- Reject non-empty initial items for V1 and V2, more than 128 items, any item
over 8,192 estimated tokens, or a combined total over 8,192 estimated tokens.
## Testing
- Cover protocol serialization, app-server forwarding, websocket and WebRTC
payloads, empty-list compatibility, version checks, and size limits.
GitOrigin-RevId: 09af367f9a07543e3f8d59b7fa5387f932dc5a6a