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

{
  "success": true,
  "data": {
    "subMerchantId": "sm-7f4a9940c621",
    "name": "Ma Boutique",
    "commercialName": "Ma Boutique SN",
    "countries": ["SN", "CI"],
    "isActive": true,
    "isDefault": true,
    "legalName": "Ma Boutique SARL",
    "businessActivity": "E-commerce",
    "managerName": "John Doe",
    "CREATED_AT": "2026-03-15T03:47:32.000Z"
  }
}