PATCH /api/v1/sub-merchant/:subMerchantId

{
  "commercialName": "New Name",
  "countries": ["SN", "CI", "BJ"]
}

{
  "success": true,
  "message": "Sub-merchant updated successfully",
  "data": {
    "subMerchantId": "sm-7f4a9940c621",
    "commercialName": "New Name",
    "countries": ["SN", "CI", "BJ"],
    "isActive": true
  }
}