fix: update test for FieldModel.format_ rename

The FieldModel enum uses format_ (with underscore) instead of format
because 'format' is a Python builtin. The string value is still 'format'.

Author: BaiyuScope3

tests/test_type_coercion.py

@@ -134,7 +134,7 @@ def test_fields_accepts_string_list(self):
         assert len(req.fields) == 3
         assert req.fields[0] == FieldModel.creative_id
         assert req.fields[1] == FieldModel.name
-        assert req.fields[2] == FieldModel.format
+        assert req.fields[2] == FieldModel.format_  # Note: format_ because 'format' is a Python builtin
         assert all(isinstance(x, FieldModel) for x in req.fields)
 
     def test_fields_accepts_enum_list(self):