Get stored prompt and completion content for a generation

Example response

{
  "data": {
    "input": {
      "messages": [
        {
          "content": "What is the meaning of life?",
          "role": "user"
        }
      ]
    },
    "output": {
      "completion": "The meaning of life is a philosophical question...",
      "reasoning": null
    }
  }
}