## Why
Resuming a running thread with a summary initial page cloned every active-turn item before discarding those outside the summary, including potentially large reasoning payloads.
## What changed
- Snapshot the active turn using the requested `TurnItemsView`, copying only the first user message and final agent message for summaries and no items for `NotLoaded`.
- Preserve full snapshots when the resume response includes full turns, even if its initial page requests a summary.
- Share item selection between snapshots and page responses, and skip selection when a turn already has the requested view.
## Testing
Add unit coverage for summary selection, completed-turn metadata, and metadata-only snapshots. Extend running-thread resume coverage for default summaries, explicit full pages, and full turns returned alongside a summary page.
GitOrigin-RevId: 6ddcd93c44dbc8083827ac82f442b31730a4edde
Charlie Marsh
05a93a8a1d