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.
Joseph Doherty