Add getExportDataBulk() to Item model for plugin use.
- Fetches item_id, item_number, name, description, unit_price,
and pic_filename for multiple item IDs in single query
- Returns results keyed by item_id for easy lookup
- Guards against empty input array
Signed-off-by: objecttothis <17935339+objecttothis@users.noreply.github.com>