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)
             }

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/StreamRelayActorTests.cs

@@ -204,4 +204,37 @@ public class StreamRelayActorTests : TestKit
         Assert.True(channel.Reader.TryRead(out var evt));
         Assert.Equal("", evt.AttributeChanged.Value);
     }
+
+    [Fact]
+    public void ListValue_EncodesAsJsonArray()
+    {
+        // List attributes must cross the wire as JSON, not as a comma-joined display string.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var actor = Sys.ActorOf(Props.Create(() =>
+            new StreamRelayActor("corr-list", channel.Writer)));
+
+        var ts = DateTimeOffset.UtcNow;
+        actor.Tell(new AttributeValueChanged("Inst", "Path", "Tags",
+            new List<string> { "a", "b" }, "Good", ts));
+
+        Thread.Sleep(500);
+        Assert.True(channel.Reader.TryRead(out var evt));
+        Assert.Equal("[\"a\",\"b\"]", evt.AttributeChanged.Value);
+    }
+
+    [Fact]
+    public void ScalarStringValue_PassesThroughUnchanged()
+    {
+        // Scalar strings must not be double-encoded.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var actor = Sys.ActorOf(Props.Create(() =>
+            new StreamRelayActor("corr-scalar", channel.Writer)));
+
+        var ts = DateTimeOffset.UtcNow;
+        actor.Tell(new AttributeValueChanged("Inst", "Path", "Name", "x", "Good", ts));
+
+        Thread.Sleep(500);
+        Assert.True(channel.Reader.TryRead(out var evt));
+        Assert.Equal("x", evt.AttributeChanged.Value);
+    }
 }