[Bug]: `dstack server` may fall back to interactive auth methods when connecting to instances/jobs

[un-def]

Steps to reproduce

1. Create an SSH fleet.
2. Make sure password and/or keyboard-interactive auth methods are enabled on the instance side.
3. Comment out the dstack key in authorized_keys

Actual behaviour

When the publickey method fails, runner_ssh_tunnel falls back to the next method (e.g., password), asks for a password, waits for 15 seconds, and fails.

Expected behaviour

runner_ssh_tunnel should immediately fail on any user interaction request.

dstack version

0.20.15

Server logs

[11:06:29] DEBUG    dstack._internal.server.background.pipeline_tasks.base:342 Processing instances item cc9e0e59-4b3c-459a-8277-fc060095a4cd
debian@192.168.122.75's password:
[...]
[11:06:44] DEBUG    dstack._internal.core.services.ssh.tunnel:180 SSH tunnel to debian@192.168.122.75 did not open in 15 seconds
           WARNING  dstack._internal.server.background.pipeline_tasks.instances.check:279 Instance vm-0 check: reachable=False health_status=HEALTHY message='SSH or tunnel error'

Additional information

No response