Add default() filter to hetzner_volume_result deep attribute access

## Description

`hetzner_volume_result.hcloud_volume.size` on line 90 of `ansible/roles/hetzner_server/tasks/main.yml` accesses a deep nested attribute without a `| default()` filter:

```yaml
when: hetzner_volume_result.hcloud_volume.size < hetzner_volume_size | int
```

If the volume creation task was skipped or failed in an unexpected way, this would produce an Ansible error rather than a graceful skip.

## Recommended fix

```yaml
when: (hetzner_volume_result.hcloud_volume.size | default(0)) < (hetzner_volume_size | int)
```

## Priority

Low — the task is unlikely to be skipped in practice since it runs unconditionally.