[Bug]: Inconsistent `dstack offer --fleet`

[jvstme]

Steps to reproduce

1. Create a cloud backend, an empty elastic cloud fleet, and an SSH fleet.

   $ dstack fleet
    NAME           NODES  GPU  SPOT  BACKEND       PRICE  STATUS  CREATED    
    on-prem        1      -    -     ssh           -      active  2 days ago 
       instance=0         -    -     ssh (remote)  -      idle    2 days ago 
    default        0..    -    auto  *             -      active  1 min ago

2. Try running dstack offer with --fleet <ssh-fleet-name> and --fleet <cloud-fleet-name>

Actual behaviour

Without --fleet, both the idle SSH instance and the cloud offers are shown.

$ dstack offer --disk 1GB.. --memory 0.1GB.. --max-offers 3
 #  BACKEND                 RESOURCES                                    INSTANCE TYPE  PRICE         
 1  ssh (remote)            cpu=2 mem=1GB disk=38GB                      instance       $0       idle 
 2  vastai (dk-denmark)     cpu=32 mem=24GB disk=1GB gpu=RTX5090:32GB:1  28726526       $0.3869       
 3  vastai (us-california)  cpu=64 mem=64GB disk=1GB gpu=RTX5090:32GB:2  32851078       $0.6136

With --fleet <cloud-fleet-name>, the idle SSH fleet offer is filtered out.

$ dstack offer --disk 1GB.. --memory 0.1GB.. --max-offers 3 --fleet default
 #  BACKEND                 RESOURCES                                         INSTANCE TYPE  PRICE     
 1  vastai (dk-denmark)     cpu=32 mem=24GB disk=1GB gpu=RTX5090:32GB:1       28726526       $0.3869   
 2  vastai (us-california)  cpu=64 mem=64GB disk=1GB gpu=RTX5090:32GB:2       32851078       $0.6136   
 3  vastai (pl-poland)      cpu=96 mem=96GB disk=1GB gpu=RTXPRO6000WS:96GB:2  31065109       $1.8645   

However, with --fleet <ssh-fleet-name>, nothing is filtered out.

$ dstack offer --disk 1GB.. --memory 0.1GB.. --max-offers 3 --fleet on-prem
 #  BACKEND                 RESOURCES                                    INSTANCE TYPE  PRICE         
 1  ssh (remote)            cpu=2 mem=1GB disk=38GB                      instance       $0       idle 
 2  vastai (dk-denmark)     cpu=32 mem=24GB disk=1GB gpu=RTX5090:32GB:1  28726526       $0.3869       
 3  vastai (us-california)  cpu=64 mem=64GB disk=1GB gpu=RTX5090:32GB:2  32851078       $0.6136

Root cause — --fleet only filters idle instance offers, while backend offers are retrieved without taking fleets into consideration.

Expected behaviour

The behavior of --fleet is consistent for idle instances and backend offers.

Option 1

Make backend offers respect --fleet, so that specifying --fleet <ssh-fleet-name> filters out all backend offers.

Option 2

Ignore --fleet for idle instances too.

dstack version

0.20.13

Server logs

Additional information

No response