Makes this provider the company's preferred supplier and unsets any other, since only one provider can be preferred at a time. No request body is needed. The provider must be CONNECTED and have at least one active branch saved as a Zuper vendor; otherwise the request fails with 400. The response returns the updated provider; credentials are never returned. Requires supplier integration to be enabled for the company; otherwise the request fails with 400. Admin only: the API key must belong to an admin user; other roles receive 405 Access Denied.