## Why
A Multi-Agent V2 child can finish after the parent turn that spawned it, so its
successful completion needs to remain associated with that parent turn in
notifications and history.
## What changed
- Add `completed` to `SubAgentActivityKind` and emit the corresponding started
and completed item events on the spawning parent turn when a child succeeds.
- Persist completed activity as a canonical turn item in both legacy and
paginated history, and expose it through app-server history and schemas.
- Render completed activity as terminal in the TUI without counting it as a
separate tool call or tool-runtime event.
## Testing
- Cover successful completion in legacy and paginated history, late updates to
completed parent turns, TUI rendering, analytics, and rollout tracing.
- Verify that terminal child errors do not emit completed activity.
GitOrigin-RevId: 6c71eb8b10e7327611c6af84f70e294e27d94d02
b705b6b076