feat(sdk): add versioned delegate-SD-JWT golden vectors + consumer test

[vishkaty]

Summary

Adds a versioned, self-describing golden-vector set for the delegate-SD-JWT chain layer, plus a consumer test that verifies each vector against the SDK and asserts the compact-serialization wire invariants. Addresses #303.

The SDK builds delegation chains inline in chain_tests.py but ships no on-disk golden-vector artifact, so a spec/format change can silently alter the ~~-joined wire without a frozen reference catching it. These vectors are that reference.

What's included

code/sdk/python/ap2/tests/delegate_sd_jwt_vectors/

• delegate_sd_jwt_vectors.json — 4 frozen vectors, each with its public JWK, kids, verification inputs, and an expected block (hop_count, double_tilde_joins, ends_with_tilde, payload assertions).
• generate_vectors.py — the generator (pinned keys), mirroring the SDK's canonical Bank → Shopping-Agent → Credentials-Provider → Merchant flow.
• delegate_sd_jwt_vector_tests.py — consumer/verifier.

All four vectors are minted by the real ap2.sdk (sd_jwt.create / kb_sd_jwt.create / MandateClient.present) — not hand-authored strings:

vector	segments	~~ joins	ends ~
root-single-sd-jwt	1	0	✓
single-hop-payment-chain	2	1	✓
multi-hop-payment-chain	3	2	✓
single-hop-checkout-chain	2	1	✓

Verification

• Consumer test: 10 passed — asserts each vector verifies against the SDK with the expected resolved payloads, and the wire shape (single SD-JWT ends ~; an N-segment chain has exactly N−1 ~~ joins).
• Kill-tests (proving the vectors actually gate): flipping a signature byte → InvalidJWSSignature; collapsing a ~~→~ → wire-shape + verify failure. Both revert clean.
• Full SDK suite green with these added (the 2 unrelated kb_sd_jwt_intermediate_tests failures pre-exist on a pristine checkout). ruff check/format clean on the new Python.

Reproducibility

Keys are pinned and embedded, so signature verification is fully deterministic on any machine. Per RFC 9901 the compact serializations are intentionally not byte-reproducible (random disclosure salts; randomized ECDSA k) — regeneration yields semantically identical, byte-different vectors, so the committed JSON is the frozen artifact the consumer test verifies. This is documented in generate_vectors.py and the JSON's reproducibility block.

Note

Two minimal repo-config edits keep spellcheck CI green on the new files: a .cspell.json ignore-glob for the vector JSON (base64 crypto blobs) and adding sdjwt to the custom dictionary. Happy to adjust if you'd prefer a different exclusion mechanism.

Closes #303

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[vishkaty]

A note on the red Lint Code Base check: the failures are not from files in this diff. Super Linter runs Biome across the whole workspace whenever a PR touches a Biome relevant file type, and this PR includes two JSON files, so the check surfaced the preexisting Biome findings in the repo. A clean checkout of main at e1ea56d shows the same 65 errors, mostly a11y title elements in the docs assets SVGs and button type props in the web client. The five files changed here lint clean under the same Biome major version. PRs that touch only Python files skip Biome entirely, which is why the check is green on some sibling PRs and red here. I am happy to file a separate PR that clears the preexisting findings if that would be useful.

[vishkaty]

Following up on the note above: #311 clears the preexisting findings and the Lint Code Base check is green there, so once it lands this PR can be rebased to pick up a meaningful lint signal.