Do not mangle absolute or protocol-relative stylesheet URLs in scopeCss

Lauri Timmaneerequested to merge
issue/canvas-3580207:3580207-error-with-cdn into 1.x

Fixes https://www.drupal.org/i/3580207 . scopeCss() rewrote every stylesheet href as if it were relative, corrupting absolute (https://cdn.example.com/...) and protocol-relative (//cdn.example.com/...) URLs when a code component's styles were loaded from a CDN. The fix leaves absolute and protocol-relative hrefs untouched and only resolves genuinely relative ones. Covered by a new vitest unit test (ui/tests/vitest/unit/scope-css-fetch-href.test.js, 3 cases: absolute, protocol-relative, relative). Verified green locally and on the GitHub CI mirror.

Closes #3580207

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