Batch cancel rentals

    Cancel multiple rentals in one request. Accepts a flat list of rentals
    (which may span several orders); the backend groups them by order.

    TRANSACTION BEHAVIOR:
    - Each rental is cancelled in its own transaction, so one rental failing
      does not roll back the others (and never rolls back other orders).
    - Cancel is only valid for pre-on-rent rentals (Needs Approval, Draft,
      Pending, Scheduled). A rental past that point is reported "failed" with
      the invariant message, not aborted for the whole batch.
    - An already-cancelled rental is reported "skipped".

    RESPONSE:
    - Returns a per-rental result ([{rental_id, status, message}]) so the
      caller can retry only the rentals that failed.

    EMAIL:
    - Sends the existing per-rental cancellation email to the order's TAMs
      (skipping mobile-tool-trailer rentals), preserving current behaviour.
      Consolidating to one email per order is handled separately (sc-551452),
      which this endpoint enables by grouping cancelled rentals per order.