SSH shell or exec does not exit after a zone is disabled

[tobe2098]

Describe the bug
If a VM in zoneX has a multipass shell or active exec, and the zone is disabled, both instances hang indefinitely.

To Reproduce
How, and what happened?

1. multipass launch -n a --zone 2
2. multipass shell a
3. In another terminal, multipass disable-zones 2 --force
4. Shell never exits.

Expected behavior
SSH processes should abort in these cases. This bug has fixes in two layers: the first one being dropping the ssh session when an instance is disabled, the second one is making sure that SSHClient or other ssh processes in general make use of ssh_keepalive.

Additional info

• OS: Ubuntu 26.04
• CPU architecture or model: amd64
• multipass version: TC1