[Bug]: Instances created for jobs are not divided into blocks

[jvstme]

Steps to reproduce

1. Create an elastic fleet with blocks: auto.

> cat fleets/default.dstack.yml
type: fleet
name: default
nodes: ..
blocks: auto

> dstack apply -f fleets/default.dstack.yml

2. Run a job on the fleet, so that it provisions an instance that could be divided into more than one block.

> cat .dstack.yml 
type: dev-environment
ide: vscode

> dstack apply -b aws --fleet default

Actual behaviour

The provisioned instance is not divided into blocks.

> dstack fleet
 FLEET    INSTANCE  BACKEND          RESOURCES                        PRICE    STATUS  CREATED    
 default  0         aws (eu-west-3)  cpu=2 mem=8GB disk=100GB (spot)  $0.0292  busy    5 mins ago 

> dstack fleet get default --json | jq '.instances[0].total_blocks'
1

Expected behaviour

The provisioned instance is divided into blocks (in this example, 2 blocks).

dstack version

0.20.3

Server logs

Additional information

No response

[github-actions]

This issue is stale because it has been open for 30 days with no activity.

[mattate]

I ran into this same issue with SSH fleets where hosts are defined as plain strings. The root cause is in create_fleet_ssh_instance_model() in fleets.py — the if isinstance(host, str) branch hardcodes blocks = 1 instead of using spec.configuration.blocks.

Submitted a fix in #3695.