AP2 mandate support (preview)

AP2 checkout mandate support for the Shopware Agentic Commerce plugin.

This build adds the AP2 (Agent Payments Protocol) checkout-mandate flow on top of UCP checkout: a Shopware-side mandate verifier checks the AP2 mandate against the current checkout before completion, checkout responses are signed (ES256) as the merchant authorization, verified mandates are stored on the order, and the sales-channel settings gain AP2 mandate and delegated-payment capability toggles plus signing-key management.