fix(phpunit): improve method existence checks in product handling code

Extended the method existence checks throughout src/QUI/ERP/Order/Basket/ files. Improved the
robustness of the code by ensuring that methods such as 'getUuid', 'getProductSetParentUuid',
'getQuantity', 'toArticle', and 'getAttributesForUniqueField' exist before calling them, thus
preventing potential undefined method errors. Removed unnecessary comments used for type hinting in
favor of functional checks.

Related: #172