feat: MultiModel support on assistant (non stream)

[TenzDelek]

Request:

{
  "assistant_id": "acc68cb6-ea51-44a3-ad4b-6a9824a9ed70",
  "target_language": "tibetan",
  "prompt": [
    "buddha"
  ],
  "model": [
    "claude-sonnet-4-20250514","gemini-2.5-flash"
  ]
}

Response:

{
  "responses": [
    {
      "model": "claude-sonnet-4-20250514",
      "results": [
        {
          "output_text": "སངས་རྒྱས།"
        }
      ],
      "metadata": {
        "initialized_at": "2026-03-16T11:35:26.702986",
        "total_batches": 1,
        "completed_at": "2026-03-16T11:35:29.483759",
        "total_processing_time": 2.7807700634002686
      },
      "errors": []
    },
    {
      "model": "gemini-2.5-flash",
      "results": [
        {
          "output_text": "\"སངས་རྒྱས།\""
        }
      ],
      "metadata": {
        "initialized_at": "2026-03-16T11:35:26.703104",
        "total_batches": 1,
        "completed_at": "2026-03-16T11:35:27.862257",
        "total_processing_time": 1.1591510772705078
      },
      "errors": []
    }
  ]
}