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.

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScopeAccessors.cs

@@ -1,3 +1,5 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Types;
+
 namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;
 
 /// <summary>
@@ -53,7 +55,7 @@ public class AttributeAccessor
         // on the DCL round-trip for data-connected attributes. The async
         // variants (GetAsync/SetAsync) are preferred where awaiting is possible.
         get => _ctx.GetAttribute(Resolve(key)).GetAwaiter().GetResult();
-        set => _ctx.SetAttribute(Resolve(key), value?.ToString() ?? string.Empty).GetAwaiter().GetResult();
+        set => _ctx.SetAttribute(Resolve(key), AttributeValueCodec.Encode(value) ?? string.Empty).GetAwaiter().GetResult();
     }
 
     /// <summary>
@@ -70,7 +72,7 @@ public class AttributeAccessor
     /// <param name="value">The value to set.</param>
     /// <returns>A task that represents the asynchronous operation.</returns>
     public Task SetAsync(string key, object? value)
-        => _ctx.SetAttribute(Resolve(key), value?.ToString() ?? string.Empty);
+        => _ctx.SetAttribute(Resolve(key), AttributeValueCodec.Encode(value) ?? string.Empty);
 }
 
 /// <summary>