The session methods had zero coverage because they call getPayload()
(audit finding B8). Stub the payload module (secret + findByID only) and
exercise the real crypto paths: round-trip, tampered signature, swapped
payload, expired exp, malformed segments, wrong secret, jti-less token
(fail closed), post-revocation rejection, and the per-instance denylist
limit. No running Payload needed.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>