Render template preview

curl --request POST \
  --url http://localhost:8080/api/v1/client/email-templates/{id}/render-preview \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "contact_id": 123,
  "include_default_vars": true,
  "vars": {}
}
'

{
  "data": {
    "contact_id": 123,
    "html_body": "<string>",
    "include_default_vars": true,
    "preview_text": "<string>",
    "resolved_vars": {},
    "subject": "<string>",
    "template_id": 123,
    "text_body": "<string>"
  },
  "errors": [
    {
      "condition": "format",
      "error": "Invalid email format",
      "key": "email"
    }
  ],
  "message": "Operation successful",
  "meta": {},
  "success": true
}