fix(ssh): verify the host key netOrk expects, report the one seen (NetOrk/netork#1167)

The driver accepted any SSH host key. It now verifies through
napalm-device-types' HostKeyCheck (>= 4.19.0): with a known key in
optional_args["ssh_host_keys"] any other key is refused before login, and
every key seen is reported to optional_args["ssh_host_key_observer"].
Without either argument it connects as before (first use).

tests/test_vm_provision_mixin.py

@@ -616,8 +616,9 @@ def _mock_ssh_exec(exit_status: int, stdout_text: str = "", stderr_text: str = "
 
 def test_run_node_command_success_returns_stdout():
     mixin = ProxmoxVMProvisionMixin()
-    mixin._ssh_client = MagicMock()
-    mixin._ssh_client.exec_command.return_value = _mock_ssh_exec(0, stdout_text="hello\n")
+    ssh = MagicMock()
+    ssh.exec_command.return_value = _mock_ssh_exec(0, stdout_text="hello\n")
+    mixin._node_ssh = MagicMock(return_value=ssh)  # the driver's verified session
 
     result = mixin._run_node_command("echo hello", timeout=10)
 
@@ -626,10 +627,9 @@ def test_run_node_command_success_returns_stdout():
 
 def test_run_node_command_nonzero_exit_raises():
     mixin = ProxmoxVMProvisionMixin()
-    mixin._ssh_client = MagicMock()
-    mixin._ssh_client.exec_command.return_value = _mock_ssh_exec(
-        1, stderr_text="No such file or directory"
-    )
+    ssh = MagicMock()
+    ssh.exec_command.return_value = _mock_ssh_exec(1, stderr_text="No such file or directory")
+    mixin._node_ssh = MagicMock(return_value=ssh)
 
     with pytest.raises(RuntimeError, match="No such file or directory"):
         mixin._run_node_command("cat /nonexistent", timeout=10)