Fixes: 1) Improve validation logic to ensure only one default CRM provider within the model.
Extra: 1) Replace CRM object string representation to use `name` instead of `crm_lead_id`; 2) Refactor token caching to improve reliability and readability.
Fixes: 1) Resolved malformed phone number issue in serializer when `instance` is unavailable; 2) Corrected relation field naming for `OrderCrmLink` (`order_uuid` to `order`); 3) Ensured only one default CRM provider can exist.
Extra: 1) Refactored `is_business` logic with error handling; 2) Improved logging for CRM integration errors; 3) Added `integration_location` and `default` fields to CRM configuration model.
