curl --request POST \
  --url https://api.merchantops.ai/api/lakehouse/documents/{document_id}/mapping/preview-transform \
  --header 'Content-Type: application/json' \
  --data '
{
  "column_mapping": {},
  "field_rules": {},
  "group_id": "<string>",
  "policy_metadata": {
    "date_format": "MM/DD/YYYY",
    "effective_date": "<string>",
    "notes": "<string>"
  }
}
'

{
  "document_id": "<string>",
  "source_row_count": 123,
  "transformed_rows": [
    {}
  ],
  "expansion_note": "<string>"
}