This commit refactors the promotion-related serializers to improve structure, validation, and maintainability, following DRF best practices.

Key changes:
- Removed the separate `PromoValidator` class and integrated its validation logic directly into the serializers. This co-locates validation with the data representation, making the code easier to follow.
- Introduced `BasePromoSerializer` to consolidate common fields and methods, reducing code duplication across create, detail, and read-only serializers.
- Implemented custom `CountryField` and `MultiCountryField` to provide robust, reusable validation for ISO 3166-1 alpha-2 country codes.
- Simplified `PromoListQuerySerializer` by leveraging the new `MultiCountryField` and inheriting from the base pagination serializer, removing manual parameter validation.

This commit improves the validation logic within the promo serializers and refines how promotion data is handled and displayed.

Key changes:
- **Improved Validation:** The validation logic now correctly handles partial updates (`PATCH`) by checking the `mode` from the instance if it's not provided in the request data. It also prevents `max_count` from being set lower than the current `used_count` for a promo.
- **Accurate Used Count:** The `get_used_codes_count` method on the `Promo` model now correctly returns the `used_count` for promotions in `COMMON` mode.