[taran-dev4u]

ODT table-of-content containers (text:table-of-content and related index elements) were matching the unhandled element branch in _add_odf_child(), dropping the index title and entry paragraphs from the converted document.

This change visits text:index-body children for table-of-content and other index containers, routing entries to ContentLayer.BODY in source order (or ContentLayer.INVISIBLE when text:display="none" is set).

Issue resolved by this Pull Request:
Resolves #4749

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

Review evidence:

• Before: with an ODT containing a generated table of contents, the index title and all entry paragraphs were dropped, exporting only surrounding body paragraphs.
• After: index title and entries are extracted in reading order as text items in ContentLayer.BODY.
• Tests: added unit tests in tests/test_backend_opendocument.py covering table-of-contents extraction, hidden TOCs, and illustration/table index containers. All 64 tests in test_backend_opendocument.py pass.
• Checks: ruff check and ruff format --check pass on changed files.

[github-actions]

✅ DCO Check Passed

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🟢 Enforce conventional commit

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[taran-dev4u]

Closing in favor of #4750 by @ryanmeowy.