## Why
Clients could discover threads with paginated history, but could not page their
turns through `thread/turns/list`.
## What changed
- Route `thread/turns/list` for paginated threads through the thread store,
preserving cursors, sort direction, turn status, timing, errors, and the
requested `summary` or `notLoaded` item view.
- Reject the `full` turn item view for paginated threads and direct clients to
`thread/items/list` for complete item pagination.
- Return each item page entry with both its containing `turnId` and `item`, so
clients can regroup thread-wide pages into turns.
- Expose local thread-store turn and item pagination through the `ThreadStore`
implementation.
## Testing
Added app-server coverage for paging projected turns and items, item views,
cursors, turn metadata, and rejection of the unsupported `full` view.
GitOrigin-RevId: 9c4a127a9b4fa91313f0e87494c62fe2e3a06f19