fix(sdjwt): enforce aud/nonce binding on every KB hop, fail closed on terminal

[SashaMIT]

Summary

In plain terms: a presentation token's aud/nonce are what bind it to a specific recipient and session. Two gaps in KB-SD-JWT verification weakened that binding:

1. Intermediate hops never checked aud/nonce even when the caller passed expected values — the check only ran on the terminal hop. (The repo's own test_verify_rejects_aud_mismatch / test_verify_rejects_nonce_mismatch were failing on main because of exactly this.)
2. A terminal hop carrying aud/nonce verified fine with no expected values passed at all. Per RFC 9901 §7.3 a verifier MUST confirm a key-binding token's aud identifies itself; the AP2 profile always issues terminal hops with aud/nonce, and exp is optional — so a verifier that forgot to bind turned every captured presentation into a replayable bearer credential, potentially indefinitely.

Fix (kb_sd_jwt.verify)

• expected_aud/expected_nonce are now honored on every hop when provided (fixes the two failing tests).
• A terminal hop that carries aud/nonce but is verified without binding them raises (fail-closed on the spec MUST). Root-only credential verification is unaffected.

Test plan

• Full suite: 190 passed (baseline 186 passed + 2 pre-existing failures; the 2 failures are now fixed)
• 7 existing tests that verified terminal hops without binding were updated to bind expected_aud/expected_nonce — the previously-unsafe pattern
• 2 new regression tests: terminal hop carrying aud/nonce rejects an unbound verifier

Compatibility note

Callers who verified terminal presentations without passing expected_aud/expected_nonce will now get an error instead of a silent pass — that is the intended behavior change; those verifiers were accepting replays.

Made with Cursor

Made with Cursor

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CI note: the Lint Code Base failure is the repo-wide Biome run (75 errors in code/web-client TS files, also failing on main at run 25120930358). This PR touches Python SDK + tests only, so none of the reported diagnostics come from this change. The remaining blocker here is cla/google.