verify: verify_record leaves as TypeError when runtime.nonce is non-ASCII and expected_nonce is set #381

What is wrong

verify_record documents its contract as InvalidSignature for a bad signature and ValueError for every other rejection. With expected_nonce supplied and a runtime.nonce outside ASCII, it leaves as a TypeError instead:

TypeError: comparing strings with non-ASCII characters is not supported

sign.py:712 compares the two nonces with hmac.compare_digest(actual_nonce, expected_nonce), which refuses str operands containing non-ASCII characters. The schema types runtime.nonce as a string with a description saying base64url, and no pattern, so the record is schema-valid.

Reproducer, on the base record from #379 (same BASE, KEY, NOW):

body = json.loads(json.dumps(BASE)); body["runtime"]["nonce"] = "nønce"
verify_record(sign_record(body, KEY), key_to_jwk(KEY), now=NOW, expected_nonce="a-nonce-value")
# TypeError: comparing strings with non-ASCII characters is not supported

The rule it tests: spec §3.2.2 "When a challenge nonce was issued, a record that omits or mismatches it MUST be rejected", and verify_record's own exception contract.

Spec section or file

• src/agentrust_trace/sign.py:710-713
• spec/trace-v0.2.md §3.2.2 (freshness, challenge-nonce binding) and §3.3 step 2

Expected behavior

ValueError("record runtime.nonce does not match expected_nonce"), reached by comparing the UTF-8 encodings (compare_digest(actual.encode(), expected.encode())) so the comparison stays constant-time and accepts any string. Alternatively the schema constrains nonce with a base64url pattern and the record is rejected earlier as schema-invalid; either way the outcome is one of the documented ones.

Impact

A caller that catches the documented exceptions and treats anything else as a verifier fault gets a fault from attacker-influenced input: the nonce is copied from the record. The record is rejected either way, so this is a contract escape rather than a wrong verdict.

Found by the differential run in #376 (mutation m41).