Refactor the pricing calculation service and shipping rate import logic to improve consistency and reliability. This includes updating direction naming conventions, enhancing model casting, and adding comprehensive test suites.
- Update `ShippingRatesImport` to use 'outbound' and 'inbound' instead of 'export' and 'import'
- Refactor `PriceCalculatorService` to use a more modular calculation structure
- Update `ShippingRate` model to use explicit property casting for zones
- Add `HasFactory` trait to `Country` and `ShippingRate` models
- Add new database factories for `Country` and `ShippingRate`
- Implement new feature tests for API endpoints and service logic
- Add new service tests for `PriceCalculatorService`
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