fix(squash): use source='collection_code' instead of validate() override

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kdmccormick

src/openedx_content/applets/backup_restore/serializers.py

@@ -156,16 +156,12 @@ class CollectionSerializer(serializers.Serializer):  # pylint: disable=abstract-
     Serializer for collections.
     """
     title = serializers.CharField(required=True)
-    # Note: the model field is now Collection.collection_code, but the archive
-    # format still uses "key".  A future v2 format may align the name.
-    key = serializers.CharField(required=True)
+    # The model field is now Collection.collection_code, but the archive format
+    # still uses "key".  A future v2 format may align the name.
+    key = serializers.CharField(required=True, source="collection_code")
     description = serializers.CharField(required=True, allow_blank=True)
     entities = serializers.ListField(
         child=serializers.CharField(),
         required=True,
         allow_empty=True,
     )
-
-    def validate(self, attrs):
-        attrs["collection_code"] = attrs.pop("key")
-        return attrs