HTMLSanitizer test failiures due to serialization changes

`HtmlSanitizerTest | use image loading placeholder` fails on new chrome version (`139.0.7258.66`)

- [x] Do smoke test for sanitizer
- [x] Check that replacement image is shown correctly
  - [x] as `src` attribute
  - [x] as `background-image` CSS attribute
- [x] Test everywhere
  - [x] iOS app
  - [x] Android app
  - [x] Desktop app
  - [x] Chrome
  - [x] Firefox
  - [x] Safari