Fix licensed variable products showing as sold out (v0.5.11)

- Fixed is_purchasable() method in LicensedVariableProduct to delegate to
  parent WC_Product_Variable instead of checking for price (variable products
  don't have direct prices, only their variations do)
- Fixed getProductClass() filter to accept all 4 WooCommerce parameters
  and use product_id for reliable variation parent detection
- Fallback to global $post when product_id not available for backwards compat

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Product/LicensedVariableProduct.php

@@ -50,11 +50,14 @@ class LicensedVariableProduct extends WC_Product_Variable
     }
 
     /**
-     * Licensed products are purchasable
+     * Licensed variable products are purchasable if the parent check passes
+     * Variable products don't have a direct price - their variations do
      */
     public function is_purchasable(): bool
     {
-        return $this->exists() && $this->get_price() !== '';
+        // Use the parent WC_Product_Variable logic
+        // which checks exists() and status, not price
+        return parent::is_purchasable();
     }
 
     /**