[snapshot][info] Snapshot size not provided

**Describe the bug**
Snapshot size is not populated. The field is unused by the daemon.

**To Reproduce**
How, and what happened?
1. `multipass launch -n a`
2. ` multipass stop a`
3. `multipass snapshot a`
4. `multipass info --snapshots --format json`

**Expected behavior**
The field should either not be present or be populated.

**Additional info**
- OS: Ubuntu 24.04
- CPU architecture or model: amd64
- `multipass version`: 1.17.0-dev

Reproduce the issue with `multipass launch -n a`, `multipass stop a`, `multipass snapshot a`, and `multipass info --snapshots --format json`, then trace where snapshot information is assembled. Confirm whether the snapshot size should be populated or omitted, and verify that the resulting JSON matches the selected behavior.

Written by the indexing model from the issue text.