Add Product/SKU validation in Create Inventory Count Import Item API

[ravilodhi]

API Endpoint: /rest/s1/inventory-cycle-count/cycleCounts/${inventoryCountImportId}/items/add

1. If productId is passed, check whether a product exists with that ID. Return an error if no matching product is found.
2. If no productId is passed, check whether idType and idValue are provided. If they are, verify whether a valid good identification exists. Return an error if it does not.
3. If neither productId nor idType/idValue is provided, return an error.

This approach ensures clarity and prevents confusion when adding items that do not exist in the system.