Adds an OPC UA | MxGateway protocol dropdown (create-time; locked read-only on
edit), branches the primary/backup endpoint editors, serializer, and validator
by protocol, and persists DataConnection.Protocol accordingly. Updates form
tests: protocol dropdown present on create + MxGateway save round-trips typed
JSON with Protocol=MxGateway.
Joseph Doherty
be32e4a7ff