Apache Airflow SFTP provider: Async SFTP hook disables host-key verification by default

The Apache Airflow SFTP provider's deferrable code paths (`SFTPTrigger`, `SFTPHookAsync`, `SFTPClientPool`) disable SSH host-key verification by default unless the connection explicitly sets `no_host_key_check` to false. Deployments that run SFTP sensors or operators in deferrable mode are affected. An attacker in a machine-in-the-middle position between the triggerer and the SFTP server can impersonate the server and capture the credentials or data exchanged.

Users of apache-airflow-providers-sftp are recommended to upgrade to apache-airflow-providers-sftp 7.0.0 or later. This is a separate defect from CVE-2026-94134 (the synchronous SSH hook default, which also affects the SFTP hook); users who applied the fix for one also need this release for the other - apache-airflow-providers-sftp 7.0.0 fixes both.