Fix inconsistent marker boundary order v2

[niegowski]

🚀 Summary

Fixes incorrect order of adjacent marker boundary elements in editing downcast.

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📌 Related issues

• Closes Incorrect order of adjacent marker boundary elements in editing downcast #19975

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💡 Additional information

Code from this PR was previously merged and reverted because of invalid sorts of markers. In this PR, the sort was fixed.

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🧾 Checklists

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• Is the changelog entry intentionally omitted?
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[pomek]

@niegowski, could you confirm that tests from Commercial are green, too? At least, AI and TC.

[niegowski]

@niegowski, could you confirm that tests from Commercial are green, too? At least, AI and TC.

It reqired a slight change in TC test: https://github.com/ckeditor/ckeditor5-commercial/pull/9773

[pomek]

Thanks @niegowski. It's good to see that now we are prepared :D

[f1ames]

Looking good 👍

Just some minor docs-related suggestions.

[f1ames]

👍