Use this when the user explicitly asks to complete an open workflow or user-generated task. Returns the updated task. Only tasks with type workflow or user_generated can be completed through this endpoint. Other task types are managed by their owning workflow and cannot be completed here, including required-question, review, identity-verification, acknowledgment, approval, reminder, connection-advancement, entity-access, and history tasks. Calling this endpoint for an already completed supported task is idempotent. Workflow tasks still enforce assignment and required-answer restrictions; user-generated tasks enforce their normal state-update permissions.