Method mark_cancelled

src/context/task_storage.rs:471–496

Transition a task to Cancelled status, setting `completed_at` to now. Works for both RUNNING and QUEUED tasks. Returns an error if the task is not found or is already in a terminal state.

(
        &self,
        id: &str,
    )

    /// Works for both RUNNING and QUEUED tasks. Returns an error if the task
    /// is not found or is already in a terminal state.
    pub async fn mark_cancelled(
        &self,
        id: &str,
    ) -> Result<Task, Box<dyn std::error::Error + Send + Sync>> {
        let conn = Arc::clone(&self.conn);
        let id = id.to_string();
        tokio::task::spawn_blocking(move || {
            let conn = conn.lock().map_err(|e| format!("Lock error: {e}"))?;
            let now = chrono::Utc::now().to_rfc3339();
            let rows_affected = conn.execute(
                "UPDATE tasks SET status = 'CANCELLED', completed_at = ?1 WHERE id = ?2 AND status IN ('RUNNING', 'QUEUED')",
                rusqlite::params![now, id],
            )?;
            if rows_affected == 0 {
                // Check if task exists — if so, it's already terminal
                let task = read_task_by_id(&conn, &id)?;
                return Err(format!(
                    "Task {} is already in terminal state: {}",
                    id,
                    task_status_to_str(&task.status)
                ).into());
            }
            read_task_by_id(&conn, &id)
        })
        .await?
    }

    /// Reset a task to Queued status, clearing `started_at`, `completed_at`, and `error_message`.
    pub async fn reset_to_queued(