Downcast skip-level lists to valid HTML

[mmotyczynska]

🚀 Summary

A brief summary of what this PR changes.

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📌 Related issues

• Closes https://github.com/ckeditor/ckeditor5-commercial/issues/9702

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💡 Additional information

Optional: Notes on decisions, edge cases, or anything helpful for reviewers.

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🧾 Checklists

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Author checklist

• Is the changelog entry intentionally omitted?
• Is the change backward-compatible?
• Have you considered the impact on different editor setups and core interactions? (e.g., classic/inline/multi-root/many editors, typing, selection, paste, tables, lists, images, collaboration, pagination)
• Has the change been manually verified in the relevant setups?
• Does this change affect any of the above?
• Is performance impacted?
• Is accessibility affected?
• Have tests been added that fail without this change (against regression)?
• Have the API documentation, guides, feature digest, and related feature sections been updated where needed?
• Have metadata files (ckeditor5-metadata.json) been updated if needed?
• Are there any changes the team should be informed about (e.g. architectural, difficult to revert in future versions or having impact on other features)?
• Were these changes documented (in Logbook)?

Reviewer checklist

• PR description explains the changes and the chosen approach (especially when performance, API, or UX is affected).
• The changelog entry is clear, user‑ or integrator-facing, and it describes any breaking changes.
• All new external dependencies have been approved and mentioned in LICENSE.md (if any).
• All human-readable, translateable strings in this PR been introduced using t() (if any).
• I manually verified the change (e.g., in manual tests or documentation).
• The target branch is correct.

[mmotyczynska]

Note: a follow-up PR refines the intermediate wrapper type logic — it uses the type of a real sibling at that indent if one exists (so the intermediate merges with it), otherwise the ancestor's type (so all intermediates at that level agree). This fixes merging for both mixed-type siblings and deeply nested skip-level items.

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