Cancel task

Cancels a task by marking it for cancellation in the system. This method initiates task cancellation based on the task's current state: - If the task has not been sent to an agent, it cancels immediately and transitions the task to a terminal state (`STATUS_DONE_NOT_OK` with `ERROR_CODE_CANCELLED`). - If the task has already been sent to an agent, the cancellation request is routed to the agent. The agent is then responsible for deciding whether cancellation is possible or not: - If the task can be cancelled, the agent must use `UpdateTaskStatus` and set the task status to `STATUS_DONE_NOT_OK`. - If the task cannot be cancelled, the agent must use `UpdateTaskStatus` to attach a `TaskError` to the task with the error code `ERROR_CODE_REJECTED` and a `message` explaining why the task cannot be cancelled.