[dawidskowron]

Fixes #4271.

Summary

getSemanticHTML() drops CSS classes when rebuilding list items. As a result, changing a list item's alignment does not change the exported HTML, so applications saving that HTML lose the formatting.

Preserve classes on real list items, excluding ql-indent-* because semantic HTML already represents indentation through nested lists. Escape class attribute values with the existing escapeText helper, and keep empty wrapper items for skipped indentation levels classless.

Example

For a centered bullet item followed by an unaligned item:

<!-- Before -->
<ul><li>one</li><li>two</li></ul>

<!-- After -->
<ul><li class="ql-align-center">one</li><li>two</li></ul>

Tests

• Added seven regression cases covering aligned bullet and ordered lists, checklists, skipped indentation levels, indent-only items, direction and custom classes, and class attribute escaping. The bullet case also checks that classes do not leak between items. Six cases fail before the fix due to missing classes; all seven pass afterward.
• All 528 Chromium unit tests pass.
• E2E completes without unexpected failures.
• ESLint and TypeScript checks pass.