AP2 v2 cross-SDK conformance — verify against the official Python SDK in CI

[dzuluaga]

Companion to #39 (Node/TS AP2 v2 wire format). Part of the trust-level roadmap (#16).

Goal. Prove v2 conformance bidirectionally against Google's official ap2 Python SDK rather than only against our own tests. "Python support" here means interop, not a reimplementation — the Python SDK is the reference verifier.

Scope:

• CI job (Python 3.12 + ap2 SDK) that runs a conformance suite:
  • TS → Python: chains signed by attestomcp-gate (AP2 v2 conformance (Node/TS) — SD-JWT/KB-SD-JWT mandate chain wire format #39) verify with MandateClient.verify + PaymentMandateChain.parse(...).verify(...) — zero violations.
  • Python → TS: chains produced by the Python SDK (root SD-JWT + hop + closed PaymentMandate) verify with our TS chain verifier, including aud/nonce, sd_hash vs issuer_jwt_hash modes, and constraint checks.
  • Receipts: references (sha256(closed leaf JWT)) computed by each side agree.
  • Negative vectors: tampered amount, wrong nonce, forged hop → refused by BOTH sides.
• Publish the fixture vectors (chains + keys) in-repo so failures are reproducible offline.
• Track upstream SDK releases (pin + renovate) so drift from the AP2 spec surfaces in CI, not in the field.

Stretch: contribute the TS chain layer upstream to google-agentic-commerce/AP2 as a community TypeScript SDK — the AP2 team has engaged directly with this project and there is no official TS SDK today.

Depends on: #39 (nothing to conformance-test until we emit v2 chains).

[dzuluaga]

Hey @TheBlackBit, PTAL. Let's migrate this one when we complete HNP.