fix: default to iso

Author: HongleiGu

evaluation_function/evaluation.py

@@ -649,7 +649,7 @@ def _err(msg: str) -> Result:
         eval_params_dict['evaluation_type'] = answer_dict['evaluation_type']
     else:
         # return _err("evaluation_type is required in answer object")
-        eval_params_dict['evaluation_type'] = None
+        eval_params_dict['evaluation_type'] = "isomorphism"  # default to isomorphism for backward compatibility with old tests that don't specify it
 
     # Get graph structure flags from answer (with defaults)
     eval_params_dict['directed'] = ans.directed if ans.directed is not None else False