Cart and checkout validation adds billing address and PO number error targets

Shopify's API version 2026-04 adds billing address and purchase order (PO) number support to Cart and Checkout Validation Functions. Developers can now access billingAddress and poNumber fields in the function input graph and use new checkout field targets to display field-level errors. This enables server-side enforcement of compliance rules like blocking prohibited billing countries or requiring PO numbers for B2B orders, without needing client-side UI extensions. No breaking changes are introduced.