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

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.

The fix uses spec.configuration.blocks instead of the hardcoded value, consistent
with how the else branch reads host.blocks for SSHHostParams objects.

Fixes #3475

Author: mattate

src/dstack/_internal/server/services/fleets.py

@@ -684,7 +684,7 @@ async def create_fleet_ssh_instance_model(
         port = ssh_params.port
         proxy_jump = ssh_params.proxy_jump
         internal_ip = None
-        blocks = 1
+        blocks = spec.configuration.blocks
     else:
         hostname = host.hostname
         ssh_user = host.user or ssh_params.user