feat(dcl): register MxGateway protocol in factory + config flatten + options

DataConnectionFactory registers 'MxGateway' -> MxGatewayDataConnection over the
real client; AddDataConnectionLayer binds MxGatewayGlobalOptions; DeploymentManager
FlattenConnectionConfig gains an MxGateway arm using the typed serializer. Factory
test confirms Create("MxGateway") returns the adapter.

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs

@@ -767,6 +767,12 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
             return Commons.Serialization.OpcUaEndpointConfigSerializer.ToFlatDict(config);
         }
 
+        if (string.Equals(protocol, "MxGateway", StringComparison.OrdinalIgnoreCase))
+        {
+            var config = Commons.Serialization.MxGatewayEndpointConfigSerializer.Deserialize(json);
+            return Commons.Serialization.MxGatewayEndpointConfigSerializer.ToFlatDict(config);
+        }
+
         // Fallback: assume legacy flat-dict shape for any future / unknown protocol.
         try
         {