Update task status handling to reset runner_id on job cancellation and failure

- Modified SQL queries in multiple functions to set runner_id to NULL when updating task statuses for cancelled jobs and failed tasks.
- Ensured that tasks are properly marked as failed with the correct error messages and updated completion timestamps.
- Improved handling of task statuses to prevent potential issues with task assignment and execution.

internal/manager/jobs.go

@@ -944,7 +944,7 @@ func (s *Manager) handleCancelJob(w http.ResponseWriter, r *http.Request) {
 
 		// Cancel all pending tasks
 		_, err = conn.Exec(
-			`UPDATE tasks SET status = ? WHERE job_id = ? AND status = ?`,
+			`UPDATE tasks SET status = ?, runner_id = NULL WHERE job_id = ? AND status = ?`,
 			types.TaskStatusFailed, jobID, types.TaskStatusPending,
 		)
 		return err