feat(transport): round-trip ElementDataType for List attributes

Add DataType? ElementDataType to TemplateAttributeDto (optional, default null
for backward-compat with old bundles). Map it in both directions in
EntitySerializer (export + FromBundleContent) and in all three
TemplateAttribute construction sites in BundleImporter (BuildTemplate,
SyncTemplateAttributesAsync add-path, and SyncTemplateAttributesAsync
update-path including change-detection). Two new round-trip tests in
EntitySerializerTests confirm List attributes survive export→import and that
old DTOs with null ElementDataType import cleanly.

src/ZB.MOM.WW.ScadaBridge.Transport/Serialization/EntityDtos.cs

@@ -80,7 +80,8 @@ public sealed record TemplateAttributeDto(
     DataType DataType,
     bool IsLocked,
     string? Description,
-    string? DataSourceReference);
+    string? DataSourceReference,
+    DataType? ElementDataType = null);
 
 public sealed record TemplateAlarmDto(
     string Name,