Function get_check_method 

pub fn get_check_method(strict_mode: StrictHostKeyChecking) -> ServerCheckMethod

Create a ServerCheckMethod based on strict host key checking mode.

This is a pure mapping with no filesystem side effects. A missing known_hosts file is handled at verification time: russh treats an unreadable file as empty, so strict mode rejects unknown hosts and accept-new mode records them, creating the file on first recording.