fix(browser): cleanup frame-ancestors from CSP header at coverage middleware

[userquin]

Description

Running pnpm vitest --coverage from test/browser folder cannot display coverage page (iframe) in the UI, we need to cleanup frame-ancestors.

(removing the reader doesn't work)

  server: {
    headers: {
      'x-custom': 'hello',
      // Vitest iframe should still be loaded
      'X-Frame-Options': 'DENY',
      'content-security-policy': 'frame-src https://example.com; frame-ancestors https://example.com',
    },
  },

We should move the logic in this PR to utils.ts, being used a few times (DRY).

Resolves #issue-number

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