adapter.xml: Fix deserialization for Extension

Currently, the XML deserialization missed the
`<aas:refersTo>` wrapper around the single references
inside `Extension.refers_to`. This commit fixes that.

basyx/aas/adapter/xml/xml_deserialization.py

@@ -692,8 +692,10 @@ def construct_extension(cls, element: etree.Element, object_class=model.Extensio
         value = _get_text_or_none(element.find(NS_AAS + "value"))
         if value is not None:
             extension.value = model.datatypes.from_xsd(value, extension.value_type)
-        extension.refers_to = _failsafe_construct_multiple(element.findall(NS_AAS + "refersTo"),
-                                                           cls._construct_referable_reference, cls.failsafe)
+        extension.refers_to = _failsafe_construct_multiple(
+            element.find(NS_AAS + "refersTo").findall(NS_AAS + "reference"),
+            cls._construct_referable_reference, cls.failsafe
+        )
         cls._amend_abstract_attributes(extension, element)
         return extension