Re-enable SubscriptionPanel action button tests when migrating to VTL

## Context

Tests in `tests-ui/tests/platform/cloud/subscription/components/SubscriptionPanel.test.ts` have been skipped pending migration to Vue Testing Library (VTL).

## Details

The `action buttons` and `loading states` test suites are currently using `describe.skip` because the current test setup cannot find buttons by user-visible content after the PrimeVue to custom Button component migration.

**Location:** `tests-ui/tests/platform/cloud/subscription/components/SubscriptionPanel.test.ts:276`

**TODO Comment:**
```
// TODO: Re-enable when migrating to VTL so we can find by user visible content.
```

## Tasks

- [ ] Migrate test setup to Vue Testing Library
- [ ] Update button selectors to find by user-visible content (text/labels)
- [ ] Re-enable the `action buttons` test suite
- [ ] Re-enable the `loading states` test suite
- [ ] Verify all tests pass with the new approach

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7649
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7649#discussion_r2640213960
- Requested by: @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7707-Re-enable-SubscriptionPanel-action-button-tests-when-migrating-to-VTL-2d16d73d3650814cbc72d29d6bba5e7a) by [Unito](https://www.unito.io)