test(images): improve test coverage

[danilrwx]

Description

Add unit tests that cover unpacked size handling for VirtualImage and ClusterVirtualImage flows backed by PVC.

The tests verify that:

• VirtualImage created from VirtualDiskSnapshot via PVC reports stored size from PVC capacity and unpacked size from requested PVC storage;
• the helper that calculates unpacked size prefers requested PVC size and falls back to capacity when needed;
• ClusterVirtualImage created from VirtualImage on PVC copies size fields correctly.

Why do we need it, and what problem does it solve?

There is already logic around stored and unpacked size propagation for PVC-backed images, but the branch currently changes only tests.

These tests protect the expected behavior from regressions, especially for cases where storage backend capacity differs from the requested PVC size and where size information must be propagated from VirtualImage to ClusterVirtualImage.

What is the expected result?

1. Run unit tests for VI/CVI PVC-backed source handling.
2. Verify that a PVC with requested size 10Gi and actual capacity 12Gi produces:
   • status.size.stored = 12Gi;
   • status.size.unpacked = 10Gi.
3. Verify that ClusterVirtualImage copied from VirtualImage preserves the same size fields.
4. Verify that unpacked size calculation falls back to PVC capacity when the request is absent.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: test
type: chore
summary: "Add unit tests for PVC-backed VI/CVI unpacked size handling."
impact_level: low