[Bug]: Support preview-head.js, middleware.js in @storybook/experimental-addon-test/vitest/plugin

[valentinpalkovic]

Describe the bug

Currently, the preview-head.js or the middleware.js file in your .storybook folder doesn't automatically injected into the Vitest environment. Users would have to manually provide scripts in preview-head.js using the browser.testerscripts configuration and to import CSS files manually in their vitest.setup.ts file.

Additionally, the previewHead hook in .storybook/main.js is ignored as well and isn't considered.

Reproduction steps

1. git clone https://github.com/pkp/ui-library.git
2. pnpm install
3. pnpm dlx [email protected] add @storybook/experimental-addon-test
4. Tests do not consider the scripts set up in .storybook/preview-head.js, which sets up a global.js script and an external jquery script to provide the global $ variable. Additionally, the previewHead preset in .storybook/main.js is ignored as well as the proxying of a URL via .storybook/middleware.js.

[JReinhold]

I don't think we should put effort into supporting middleware.js. I wouldn't expect it to be easy to do, it's an undocumented functionality, and I personally think that users should configure it in their Vite config if they need it.

[JReinhold]

Closing in favor of #29775