aicr verify: binary-attestation verification failure degrades to attested (not unknown)

[yuanchen8911]

Summary

aicr verify degrades to the positive attested trust level when the binary attestation verification fails, not only when it is missing. An actively-invalid or tampered binary attestation is therefore treated the same as a clean bundle without one.

Evidence

In pkg/bundler/verifier/verifier.go (binary-attestation step):

• missing binary attestation -> setTrust(TrustAttested, "...binary attestation not found (incomplete chain)")
• extractBinaryDigest error -> setTrust(TrustAttested, "could not extract binary digest...")
• VerifyBinaryAttestation(...) returns an error -> setTrust(TrustAttested, "binary attestation verification failed") and return result, nil

So a verification failure yields attested (level 3), the same as a legitimate missing-attestation case, rather than unknown/unverified.

Surfaced reviewing PR #1547.

Question / proposed fix

Decide whether a failed (as opposed to absent) binary attestation should:

• drop to unknown (treat an invalid attestation as a hard failure), or
• remain attested by design (the verifying binary may differ from the build binary, so a mismatch is tolerated).

If the latter, document the rationale; if the former, distinguish "missing" from "verification failed" and add a regression test. The PR #1547 docs were updated to say attested can mean "binary attestation missing/unverified" so the docs already match current behavior.