This commit enhances the stock item management by introducing automatic calculation of selling price and markup based on the cost price. Key changes include:
- Integration of `applyDefaultMarkupToPatch` to automatically set `selling_price` and `markup` at 80% of the cost price when they are not explicitly provided during stock item creation and updates.
- Updates to the `create` and `update` functions in the stock items service to ensure that existing values are respected and not overwritten if they are already set by the operator.
- Tests added to verify the correct behavior of auto-markup during both creation and updates of stock items, ensuring that the functionality works as intended across various scenarios.
This feature aims to streamline the pricing process for stock items, reducing manual input and potential errors.
