test 1: header-only X-Agent-Id=ai:alice
settle 2s for read-settle
  stored metadata.agent_id for header-only write: ai:alice (expected ai:alice)
test 2: body.metadata.agent_id=ai:body-wins vs X-Agent-Id=ai:attacker
settle 2s for read-settle
  stored metadata.agent_id for body+header conflict: ai:attacker