Add core WooCommerce product_brand to Pinterest feed as g:brand

[rbgly]

Summary

• Adds g:brand field to the Pinterest XML product feed, sourced from the core WooCommerce product_brand taxonomy (WooCommerce Brands)
• Variations inherit brand from their parent product, consistent with how g:product_type handles categories
• A pinterest_for_woocommerce_product_brand filter allows overriding the brand value per product

Closes PIN4WOO-68

Test plan

• Add a brand via Products → Brands and assign it to a product
• Trigger a feed sync and inspect the XML — confirm <g:brand>BrandName</g:brand> appears for that product
• Confirm products with no brand assigned have no <g:brand> element in the feed
• Confirm a product variation inherits the brand from its parent
• Run vendor/bin/phpunit --testsuite unit --filter testPropertyBrand — should show 2 passing tests

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