Get Preferred Supplier Provider

Returns the company's preferred supplier provider together with its favorite branch. While the provider is connected, that branch is the Zuper vendor used to price and pre-fill supplier materials on proposals. data is null (with the message No preferred supplier set) when no provider is preferred, and favorite_supplier_vendor is null when the provider has no active favorite branch. The preferred provider is returned even after it loses its connection, so check connection_status before relying on it. Credentials are never returned. Requires supplier integration to be enabled for the company; otherwise the request fails with 400.