fix: use fleet blocks config for string SSH hosts in create_fleet_ssh_instance_model

[mattate]

Summary

Fixes #3475

When SSH fleet hosts are specified as plain strings (e.g., hosts: ["192.168.0.1"]), the fleet-level blocks configuration is ignored and hardcoded to 1. This prevents instances from being divided into multiple GPU blocks, even when blocks: auto or a specific number is set in the fleet YAML.

Root Cause

In create_fleet_ssh_instance_model(), the if isinstance(host, str) branch hardcodes blocks = 1, while the else branch (for SSHHostParams objects) correctly reads blocks from host.blocks. The fleet-level spec.configuration.blocks is never used for string hosts.

Fix

Replace blocks = 1 with blocks = spec.configuration.blocks in the string host branch, consistent with how fleet-level configuration is propagated for other SSH parameters (ssh_user, ssh_key, port) in the same branch.

Steps to reproduce

1. Create an SSH fleet with blocks: 3 (or blocks: auto):

type: fleet
name: my-fleet
blocks: 3
ssh_config:
  user: myuser
  identity_file: ~/.ssh/id_rsa
  hosts:
    - 192.168.0.1

2. Apply: dstack apply -f fleet.dstack.yml
3. Check blocks: dstack fleet get my-fleet --json | jq ".instances[0].total_blocks"
4. Observe total_blocks: 1 instead of 3

After this fix, the instance correctly reports total_blocks: 3.

AI Assistance Disclosure

This PR was written with AI assistance (code agent).

[un-def]

Thanks for the contribution. Actually, this fix is not related to #3475 but addresses another known issue – #3278

The fix is partial, though – it works when a host is specified as a string, but the object form still defaults to blocks: 1 if ssh_config.hosts[*].blocks is not specified. I think this makes the situation worse:

Now:

blocks: 4
ssh_config:
  hosts:
    - 192.168.100.100  # blocks = 1, top-level blocks is ignored
    - hostname: 192.168.100.200  # blocks = 1, top-level blocks is ignored

With the fix:

blocks: 4
ssh_config:
  hosts:
    - 192.168.100.100  # blocks = 4, top-level blocks value is used as a fallback
    - hostname: 192.168.100.200  # blocks = 1, top-level blocks is still ignored

@peterschmidt85 I think we should reopen #3278 and make a proper fix with the following fallback chain: ssh_config.hosts[n].blocks → top-level blocks → 1

[un-def]

Closed in favor of #3700