feat: implement mock AP2 signature verification

[0x8000-0000]

Description

Why:
- To verify that the server correctly rejects invalid AP2 mandates with the appropriate error code.
- Required for conformance testing of the AP2 security flow.

What:
- Added 'Ap2VerificationError' to exceptions.
- Implemented '_verify_ap2_mandate' in 'CheckoutService' to reject mandates containing the substring 'invalid_signature'.
- Updated 'complete_checkout' to invoke this verification.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update

---

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules; this change depends on feat: implement mock AP2 signature verification samples#26