## Why
Clients that account for individual upstream Responses API calls need the exact
usage for each completion rather than accumulated or estimated thread usage.
## What changed
- Add the opt-in `rawResponse/completed` app-server notification with the
`threadId`, `turnId`, `responseId`, and upstream `usage` payload.
- Emit the transient event for normal turns and compaction requests when
`experimentalRawEvents` is enabled. The event is not persisted or replayed,
and `usage` is `null` when the upstream response omits it.
- Export the notification in the app-server schemas and document its behavior.
## Testing
- Cover exact token breakdowns for turn and compaction completions.
GitOrigin-RevId: 49040a060bf1784c1ff1dd62112efb4a778428c4
