Marks the entire task as done. This differs from PUT /tasks/{taskId}/products/{productId}/complete, which completes a single product within the task's scope — the task itself stays open until this endpoint is called directly, regardless of how many of its products are done. Use PUT /userTasks/{taskId}/revoke to undo. Fails if the task does not exist, or if you are neither its creator nor its assignee and lack the complete permission.