Add up to 25 items to a trip in one call (accommodation, activity, flight, meal, transport). Checks the Free items-per-trip limit once for the whole batch: all-or-nothing, so a batch that would exceed it creates nothing. Every item is validated and geocoded before anything is written, so one invalid item also creates nothing. Use this instead of repeated trip_item_add calls when adding several items at once — it costs one MCP request instead of one per item.