test(axon): cover missing/empty signature in default_verify

Exercise Axon.default_verify directly: a valid signature passes (nonce
recorded), an empty or absent signature now raises "Missing Signature"
(the auth-bypass regression this branch fixes), and a present-but-invalid
signature still raises "Signature mismatch".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>