[Feature Request] Add IsConnected() method to check session status

The existing IsConnected property does not always reliably reflect the actual state of the SSH session. For example, it may still return true even if the connection has silently dropped due to network issues or server-side timeouts.

It would be helpful to have a method like IsConnected() that actively attempts to communicate with the server (e.g., by sending a keep-alive packet or executing a lightweight command) to verify whether the session is still alive. This would improve robustness in long-running applications and allow developers to handle disconnections more gracefully.

Start by tracing the existing IsConnected property and the SSH session's keep-alive or lightweight-command mechanisms. Compare the proposed ways to actively verify a dropped connection and define the method's observable success and failure behavior. Done means a documented IsConnected() method reliably reports whether the session is still usable.