[Bug]: Cannot update/recreate SSH fleet

[un-def]

Steps to reproduce

1. Create and apply an SSH fleet configuration:

   type: fleet
   name: fleet-vm
   ssh_config:
     hosts:
       - hostname: 192.168.122.75
         user: debian
         identity_file: ~/.ssh/vm_debian12

   dstack apply -f .dstack/fleet-vm.dstack.yml
    Project        main
    User           admin
    Configuration  .dstack/fleet-vm.dstack.yml
    Type           fleet
    Fleet type     ssh
    Nodes          1
    Placement      cluster
   
   Fleet fleet-vm does not exist yet.
   Create the fleet? [y/n]:

2. Update and apply the configuration to add more hosts, or even apply the same exact configuration, to, for example, update the shim binary:

   type: fleet
   name: fleet-vm
   ssh_config:
     hosts:
       # the same host
       - hostname: 192.168.122.75
         user: debian
         identity_file: ~/.ssh/vm_debian12
       # a new host
       - hostname: 192.168.122.58
         user: ubuntu
         identity_file: ~/.ssh/vm_ubuntu24_04

Actual behaviour

dstack apply -f .dstack/fleet-vm.dstack.yml
Instances [192.168.122.75] are already assigned to a fleet.

Expected behaviour

No response

dstack version

master branch (0.18.22 with #1938 merged)

Server logs

No response

Additional information

No response