Migrate from Vue Test Utils to Vue Testing Library

## Context

The project currently uses Vue Test Utils for component testing. However, Vue Testing Library provides better testing practices by encouraging user-centric queries and accessible selectors that align with how users actually interact with components.

## Benefits

- **User-focused testing**: Tests verify what users see and interact with, not implementation details
- **Better accessibility**: Encourages proper use of ARIA attributes and semantic HTML
- **Query priority guidance**: Built-in best practices for selecting elements (role > label > text > testid)
- **Consistent with ecosystem**: Aligns with Testing Library philosophy used across React, Angular, and other frameworks

## References

- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7303#discussion_r2604969095
- Related PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7303
- Vue Testing Library: https://testing-library.com/docs/vue-testing-library/intro
- Testing Library Cheatsheet: https://testing-library.com/docs/vue-testing-library/cheatsheet

Requested by: @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7308-Migrate-from-Vue-Test-Utils-to-Vue-Testing-Library-2c56d73d3650812b866df90c16262c63) by [Unito](https://www.unito.io)