Printful HTTP client (v2 + v1 for sync products), Provider behaviour
implementation with all callbacks (test_connection, fetch_products,
submit_order, get_order_status, fetch_shipping_rates), and multi-provider
order routing that looks up the provider connection from the order's
product instead of hardcoding "printify".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>