The Berlin spatial data infrastructure publishes one WFS endpoint per
dataset under a common path, e.g. https://gdi.berlin.de/services/wfs/atkis
for the ATKIS layers. This helper only composes that URL; it does not
contact the server.
Value
character of length one, the service URL without query string.
Signals an error when service is not a single path element, because the
resulting URL would otherwise be handed to xml2::read_xml(), which parses
any string containing < or > as literal XML rather than fetching it.