List the connectors used by the caller's toolboxes

Lists the connectors that appear in the toolboxes GET /toolbox would return for this caller, and how many of those toolboxes hold each one. These are the options for GET /toolbox?connector_slug=, the same way GET /connector/categories is the options for GET /connector?category=. Build a connector filter on this rather than on GET /connector: the catalog offers connectors the caller has no toolbox for, and the empty list that follows selecting one reads as a bug. Filtering on any slug returned here yields exactly that row's toolbox_count toolboxes from GET /toolbox under the same ?type=, so pass ?type= here whenever you pass it there. toolbox_count counts distinct toolboxes over the caller's whole visible set, never over one page of it. Ordered most-used first, slug ascending as the tiebreak. Cursor-paginated. Visibility follows GET /toolbox: a connector reachable only through someone else's private toolbox is not listed.