fix(siteruntime): encode list attribute writes via AttributeValueCodec (was .ToString())

Replace value?.ToString() with AttributeValueCodec.Encode(value) in
AttributeAccessor indexer set and SetAsync, so a List<string>{"a","b"}
encodes to ["a","b"] instead of the garbage ToString representation.
Add using ZB.MOM.WW.ScadaBridge.Commons.Types. Tests verify the codec
contract (list→JSON array, scalar passthrough, null); full round-trip
through the accessor is not viable without a live Akka ActorSystem —
noted in-test with explanation.

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScopeAccessorTests.cs

@@ -1,3 +1,4 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Types;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Scripts;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;
 
@@ -84,4 +85,54 @@ public class ScopeAccessorTests
         var temp = children["TempSensor"];
         Assert.Equal("Motor.TempSensor", temp.Path);
     }
+
+    // --- AttributeAccessor encoding contract ----------------------------------
+    //
+    // AttributeAccessor.this[key].set and SetAsync both route through
+    // ScriptRuntimeContext.SetAttribute(name, encodedString), which requires
+    // a live Akka IActorRef; ScriptRuntimeContext has no virtual members and
+    // its constructor cannot be satisfied without a real ActorSystem, so a
+    // full-round-trip unit test through the accessor+context is not viable
+    // without a heavy Akka harness.
+    //
+    // Instead we test the encoding decision directly: AttributeAccessor is now
+    // documented to delegate value serialisation to AttributeValueCodec.Encode.
+    // These tests verify that contract at the codec level, which is exactly what
+    // the fix makes the accessor invoke.
+
+    [Fact]
+    public void AttributeValueCodec_Encode_List_ProducesJsonArray()
+    {
+        // A List<string> must encode to a JSON array, not the garbage
+        // "System.Collections.Generic.List`1[System.String]" that .ToString() produced.
+        var list = new List<string> { "a", "b" };
+        var encoded = AttributeValueCodec.Encode(list);
+        Assert.Equal("[\"a\",\"b\"]", encoded);
+    }
+
+    [Fact]
+    public void AttributeValueCodec_Encode_Scalar_PassesThrough()
+    {
+        // A plain string scalar must be returned unchanged (byte-identical to
+        // the historical value?.ToString() path for strings).
+        var encoded = AttributeValueCodec.Encode("x");
+        Assert.Equal("x", encoded);
+    }
+
+    [Fact]
+    public void AttributeValueCodec_Encode_Null_ReturnsNull()
+    {
+        // AttributeAccessor coalesces null → "" at the call site,
+        // but the codec itself must return null for null input.
+        Assert.Null(AttributeValueCodec.Encode(null));
+    }
+
+    [Fact]
+    public void AttributeValueCodec_Encode_IntList_ProducesJsonArray()
+    {
+        // Integer list elements encode via InvariantCulture IFormattable.
+        var list = new List<int> { 1, 2, 3 };
+        var encoded = AttributeValueCodec.Encode(list);
+        Assert.Equal("[\"1\",\"2\",\"3\"]", encoded);
+    }
 }