curl --request POST \
  --url https://api.example.com/v1/populations/{population_id}/retest

{
  "data": {
    "id": "d670f351-8567-4586-9bba-b81add1bebe3",
    "created_at": "2025-06-23T15:38:01Z",
    "name": "Developers",
    "public": true,
    "question_count": 42,
    "simulacrum_count": 984,
    "status": "Tested",
    "average_accuracy": 0.8528,
    "average_squared_error": 0.1607,
    "average_normalised_information_loss": 0.0063,
    "data_source": "User onboarding flow",
    "description": "A model of Developers based on the 2024 Stack Overflow Developer Survey.",
    "effective_date": "2024-01-09",
    "reality_target": "Professional software developers",
    "simulation_engine": "answers-1",
    "test_finished_at": "2023-11-07T05:31:56Z",
    "test_started_at": "2023-11-07T05:31:56Z",
    "upload_filename": "seed_data.csv"
  },
  "errors": [
    {
      "code": "<string>",
      "message": "<string>"
    }
  ]
}