Fix <mark> styling in Rich Text Editor

Chad Lavimoniererequested to merge
514326-textmarker-mark-makes-text-invisible-in-rich-text-editor into master

What does this MR do and why?

Ensure styles for <mark> elements work when shown in the RTE.

References

Screenshots or screen recordings

Before After
Screenshot_2025-08-22_at_16.09.19 Screenshot_2025-08-22_at_16.09.04

How to set up and validate locally

  1. Check out this branch
  2. Create a comment or description somewhere in the GDK using markdown. Include the following content:
thi<mark>s text has a hig</mark>hlight in it

hello
  1. Click the "switch to rich text editing" button.
  2. You should see the highlighted text and it should be styled as a highlight.

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Related to #514326 (closed)

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