evalset_result is serialized string

[marapper]

Describe the bug
While files as some_agent_evalset_correctness_1765207385.9918857.evalset_result.json are technically JSON, in fact, it is saved as a string with serialised json.

"{\"eval_set_result_id\":\"dossier_manager_agent_evalset_correctness_1765207385.9918857\",\"eval_set_result_name\":\"dossier_manager_agent_evalset_correctness_1765207385.9918857\",\

Expected behavior

It could be correct JSON to parse it in more convenient way.

{
  "eval_set_result_id": "dossier_manager_agent_evalset_correctness_1765207385.9918857",
  "eval_set_result_name": "dossier_manager_agent_evalset_correctness_1765207385.9918857",
  "eval_set_id": "evalset_correctness",
  "eval_case_results": [
    {

Desktop (please complete the following information):

• OS: macOS
• Python version(python -V): 3.11.5
• ADK version(pip show google-adk): 1.20

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-flash-lite

Additional context
None.

[surajksharma07]

@marapper The double-serialization bug is at local_eval_set_results_manager.py:57,64 where model_dump_json() returns a JSON string that gets serialized again by json.dumps().

Change line 64 to f.write(eval_set_result.model_dump_json(indent=2)) and remove line 57, or use model_dump() on line 57 and keep the json.dumps() on line 64.

Also fix line 83 to use model_validate() instead of model_validate_json() since json.load() already returns a dict.

Thanks!