Method mark_failed

src/context/task_storage.rs:443–465

Transition a task to Failed status, setting `completed_at` to now and `error_message`. Guarded: only updates tasks with status RUNNING or QUEUED. If the task exists but is already in a terminal state (COMPLETE, FAILED, or CANCELLED), returns the current task as-is (no-op).

(
        &self,
        id: &str,
        error_message: &str,
    )

    /// but is already in a terminal state (COMPLETE, FAILED, or CANCELLED), returns
    /// the current task as-is (no-op).
    pub async fn mark_failed(
        &self,
        id: &str,
        error_message: &str,
    ) -> Result<Task, Box<dyn std::error::Error + Send + Sync>> {
        let conn = Arc::clone(&self.conn);
        let id = id.to_string();
        let error_message = error_message.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 = 'FAILED', completed_at = ?1, error_message = ?2 WHERE id = ?3 AND status IN ('RUNNING', 'QUEUED')",
                rusqlite::params![now, error_message, id],
            )?;
            if rows_affected == 0 {
                // Task may not exist, or may already be in a terminal state (e.g. CANCELLED)
                return read_task_by_id(&conn, &id);
            }
            read_task_by_id(&conn, &id)
        })
        .await?
    }

    /// Transition a task to Cancelled status, setting `completed_at` to now.
    ///