Keep the complete result RPC format the same for session/load and session/prompt

[Smityz]

session/load result response is

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": null
}

session/prompt result reponse is

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "stopReason": "end_turn"
  }
}

I think session/load response should have the same result style like session/prompt , frontend also need to know why this session is stopped when loading history