Apache Airflow: incomplete fix lets a DAG author run code in the scheduler

Apache Airflow 3.3.1 shipped a fix for CVE-2026-67260 that checks the scheduler's decoded next_kwargs value is a dictionary, but performs that check only after deserializing the whole stored payload. Deserialization reconstructs nested objects of classes permitted by the deserialization allow-list — which by default includes Airflow's own classes — and runs their constructors inside the scheduler process before the check can reject the value. A Dag author, who controls a task's next_kwargs through the task execution API, can therefore still trigger constructor execution in the scheduler when the awaiting_input timeout sweep processes the task, and in the API server when the task instance is served to a worker using an older Execution API version; no non-default configuration is required. Versions before 3.3.0 are not affected. This is an incomplete fix for CVE-2026-67260: deployments that upgraded to 3.3.1 for that issue must also upgrade to apache-airflow 3.4.0 or later.