fix(contentstore): guard downstream_customized copy on paste (#38457)

Fix clipboard paste crash in upstream-link setup when pasted block does not expose `xml_attributes`.

Before change, paste flow always read `temp_xblock.xml_attributes["downstream_customized"]`. Blocks like `DragAndDropBlockWithMixins` do not have `xml_attributes`, so Studio raised `AttributeError` and paste failed.

Change now:
- Read `downstream_customized` from parsed block field first (`getattr(temp_xblock, "downstream_customized", [])`).
- Read `xml_attributes` only when block exposes it.
- Set `downstream_customized` only when block has field.

Author: navinkarkera

cms/djangoapps/contentstore/helpers.py

@@ -509,8 +509,18 @@ def _fetch_and_set_upstream_link(
             # temp_xblock.display_name == temp_xblock.upstream_display_name
             # temp_xblock.data == temp_xblock.upstream_data   # for html blocks
             # Even then we want to set `downstream_customized` value to avoid overriding user customisations on sync
-            downstream_customized = temp_xblock.xml_attributes.get("downstream_customized", '[]')
-            temp_xblock.downstream_customized = json.loads(downstream_customized)
+            downstream_customized = getattr(temp_xblock, "downstream_customized", [])
+            # XmlMixin blocks expose raw XML attrs on `xml_attributes`; other blocks (e.g. DnD)
+            # may not have this attribute, but still have parsed downstream_customized field.
+            xml_attributes = getattr(temp_xblock, "xml_attributes", None)
+            if isinstance(xml_attributes, dict):
+                raw_downstream_customized = xml_attributes.get("downstream_customized")
+                if isinstance(raw_downstream_customized, str):
+                    downstream_customized = json.loads(raw_downstream_customized)
+                elif isinstance(raw_downstream_customized, list):
+                    downstream_customized = raw_downstream_customized
+            if hasattr(temp_xblock, "downstream_customized"):
+                temp_xblock.downstream_customized = downstream_customized
 
 
 def _import_xml_node_to_parent(