[python] fix setting property on flattened property when flatten property is not called properties (#8867)

Co-authored-by: iscai-msft <isabellavcai@gmail.com>

Author: iscai-msft

.chronus/changes/python-fixFlattenProperties-2025-9-30-12-5-48.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix setting attribute of flattened property when flattened property's name is not `properties`

packages/http-client-python/generator/pygen/codegen/templates/model_dpg.py.jinja2

@@ -78,7 +78,7 @@
         if key in self.__flattened_items:
             if self.{{ flattened_property_attr }} is None:
                 self.{{ flattened_property_attr }} = self._attr_to_rest_field["{{ flattened_property_attr }}"]._class_type()
-            setattr(self.properties, key, value)
+            setattr(self.{{ flattened_property_attr }}, key, value)
         else:
             super().__setattr__(key, value)
         {% endif %}