Simpler variant matching for single-variant products

Variant matching is now more intuitive when working with products that have only one variant – you no longer need to include a variant ID column like SKU or Barcode.

🔧 Improvements

• Single-variant products – automatic variant matching when a product has one variant, so you don’t need Variant ID, SKU, Barcode, or Option columns in your import file.
• Better error messages – attempting to update a multi-variant product without a variant ID column now shows a clear PRD009 error explaining which columns are required.