Records the connected user's review sign-off on a completed task. Requires an OAuth connection made by a
named user; API keys are rejected with HTTP 403. The task must be completed first (HTTP 409 otherwise).
Only a reviewer on the task who has not signed off yet may sign off (HTTP 403 otherwise); when the
organization enforces an ordered reviewer queue, it must also be their turn. A fully signed-off task is
rejected with HTTP 409. Returns the updated task.