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/EntitySerializer.cs

@@ -51,7 +51,8 @@ public sealed class EntitySerializer
                         DataType: a.DataType,
                         IsLocked: a.IsLocked,
                         Description: a.Description,
-                        DataSourceReference: a.DataSourceReference)).ToList(),
+                        DataSourceReference: a.DataSourceReference,
+                        ElementDataType: a.ElementDataType)).ToList(),
                     Alarms: t.Alarms.Select(a => new TemplateAlarmDto(
                         Name: a.Name,
                         Description: a.Description,
@@ -203,6 +204,7 @@ public sealed class EntitySerializer
                         IsLocked = a.IsLocked,
                         Description = a.Description,
                         DataSourceReference = a.DataSourceReference,
+                        ElementDataType = a.ElementDataType,
                     });
                 }
                 foreach (var al in dto.Alarms)