Generalize the browse-button gate from IsOpcUa to IsBrowsable (OPC UA or
MxGateway, both implement IBrowsableDataConnection site-side). The generalized
NodeBrowserDialog + BrowseNodeCommand path already routes by protocol; non-
browsable protocols return NotBrowsable. Test Bindings stays OPC-UA-only (its
read path is out of this slice's scope).
Joseph Doherty
569edf2975