Highlight: inherit block text color for background-only marks

[dilipom13]

What

Fixes #36532

When only a background highlight is applied, the <mark> element now uses color: inherit so block-level text colors are preserved instead of being overridden by the browser default mark color.

How

• Add getHighlightTextColorStyle() helper in packages/format-library/src/text-color/utils.js
• Apply color:inherit in the highlight format when a background color is set without an explicit text color
• Unit tests for the helper logic

Test plan

• Add a paragraph with a block-level text color in the sidebar
• Select text and apply only a background highlight (no text color)
• Confirm the text keeps the block-level color
• Apply both text and background colors — both should still work
• Clear highlight and confirm formatting is removed as expected

[github-actions]

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[dilipom13]

Hello
Could you please add the [Type] Bug label? I don't have permission to add labels on this repo.