feat(comm): stream List attribute values as canonical JSON

Replace ValueFormatter.FormatDisplayValue with AttributeValueCodec.Encode
in StreamRelayActor so List<T> attribute values cross the gRPC wire as a
JSON array (e.g. ["a","b"]) rather than a comma-joined display string.
Scalars and null values are unaffected. Tests cover List→JSON, scalar
string pass-through, and null→empty-string.

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/StreamRelayActor.cs

@@ -45,7 +45,7 @@ public class StreamRelayActor : ReceiveActor
                 InstanceUniqueName = msg.InstanceUniqueName,
                 AttributePath = msg.AttributePath,
                 AttributeName = msg.AttributeName,
-                Value = ValueFormatter.FormatDisplayValue(msg.Value),
+                Value = AttributeValueCodec.Encode(msg.Value) ?? string.Empty,
                 Quality = MapQuality(msg.Quality),
                 Timestamp = Timestamp.FromDateTimeOffset(msg.Timestamp)
             }