test(assets): add E2E coverage for bulk-delete confirmation dialog flow

## Overview

This issue tracks follow-up E2E test coverage for the **bulk (multi-select) delete confirmation** flow in the Assets sidebar, deferred from PR #10785.

PR #10785 added single-item delete confirm/cancel tests. The multi-select path was identified as missing coverage during review (see [review comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10785#discussion_r3019178048)).

## Requested by

@dante01yoon

## Scope

Add the following tests inside the `'Assets sidebar - delete confirmation'` describe block in `browser_tests/tests/sidebar/assets.spec.ts`:

1. **Bulk delete shows confirmation with selected count** — select 2+ assets via modifier-click, trigger delete (footer delete button or context menu), assert `comfyPage.confirmDialog.root` is visible and contains text matching the selected count (e.g. `/Delete\s+2\s+assets\?/i`).
2. **Confirming bulk delete removes all selected assets** — confirm the dialog, assert `tab.assetCards` count decreases by the number deleted, and a success toast (`.p-toast-message-success`) is visible.
3. **Cancelling bulk delete preserves all assets** — cancel via `comfyPage.confirmDialog.reject`, assert asset count is unchanged.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10785
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10785#discussion_r3019178048
- Related issue: #10781

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10786-test-assets-add-E2E-coverage-for-bulk-delete-confirmation-dialog-flow-3356d73d365081b7ab74d248bce4b72f) by [Unito](https://www.unito.io)