curl --request POST \
  --url https://api.sumvin.com/v0/chat/sessions/{session_id}/messages \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "messages": [
    {
      "id": "<string>",
      "role": "<string>",
      "parts": [
        {}
      ],
      "created_at": 123,
      "metadata": {}
    }
  ]
}
'

{
  "_links": {},
  "saved_count": 123
}