When increasing the size of a full disk, the root partition does not resize accordingly

**Describe the bug**
Normally, when you increase the disk size of an instance (`mp set local.instance.disk=...`), the root partition gets resized by cloud-init to use the newly available space on the disk. But when the disk is already used at full capacity, increasing the disk size will not automatically increase the root partition size.

**To Reproduce**
1. `multipass exec instance -- dd if=/dev/zero of=big_file bs=1M count=10000 # or more if needed to fill the disk`
2. `multipass stop instance`
3. `multipass set local.instance.disk=20GB # or more than the current size`
4. `multipass exec instance lsblk`
```
sda 8:0 0 20G 0 disk
├─sda1 8:1 0 4.9G 0 part /
```

**Expected behavior**
Have `/dev/sda1` expand to take up the available space in `/dev/sda`.

Reproduce the full-disk scenario with the listed multipass, dd, stop, disk-resize, and lsblk commands, then inspect the instance disk-resize path and its cloud-init interaction. Done means that after increasing the disk, lsblk shows /dev/sda1 expanded to use the available space on /dev/sda.

Written by the indexing model from the issue text.