fix(browser): remove enforce from vitest:browser plugin to simplify custom middleware plugin usage

[hi-ogawa]

Description

Close #3743

I wasn't sure if Vitest side change is worth it for this issue since currently user-side workaround exists, but I created this PR as this demonstrates the reproduction and allows quick experiment.

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Actually crossOriginIsolated might not be working for some browsers on CI. I will look into this further.

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[hi-ogawa]

I suppose user-side workaround suffices and committing coop/coep tests might be an unnecessary burden.
Also, I cannot setup playwright/webkit locally on my archlinux PC, so it's hard to debug the issue.

I'm closing this PR for now, but feel free to pick up changes if it's considered important.