Explicit `undefined` keys on optional snapshot fields were converted
to `null` by `ensureValidJson`, failing `AgentSnapshotPayloadSchema`
validation on the wire. Stop writing those keys so they remain absent,
which is what `.optional()` expects.
Also adds a test that parses `list_agents` output through the schema
to catch this class of bug.
Mohamed Boudra
a2b743a6d3