Implement AP2 v0.2.0 mandate chaining using the Python SDK so an agent application can build a verifiable IntentMandate→CartMandate→PaymentMandate chain end-to-end

Verified steps

1. Install the AP2 Python SDK from PyPI (pip install ap2-sdk); the v0.2.0 release shipped April 2026 and is the current stable version; check the repo at github.com/google-agentic-commerce/AP2 for the latest changelog before installing.
2. Build an IntentMandate using the ap2.models.IntentMandate Pydantic class, specifying budget ceiling, allowed payment instruments, merchant category constraints, and expiry timestamp; sign it with the user's private key using sdk.sign_mandate(mandate, private_key).
3. When the agent selects a merchant cart, call the merchant's UCP endpoint to receive a CartMandate; use sdk.verify_cart_mandate(cart_mandate, merchant_public_key) to verify the merchant's ECDSA signature before proceeding.
4. Construct a PaymentMandate linking the CartMandate ID and IntentMandate ID; call sdk.create_payment_mandate(cart_mandate_id, intent_mandate_id, payment_instrument) and sign with the user's key.
5. Submit the three-mandate chain to the credential provider's verify_and_pay endpoint; the provider validates the full chain signature integrity, intent constraints, and cart immutability before authorizing settlement.