PDF not having hierarchial structure.

### Bug
When processing a .docx file with the simple pipeline, the doctags output correctly preserves the hierarchical structure of headings (section_header_level_1 through section_header_level_4).

However, when the same document is converted to .pdf and processed using pdfpipeline(), the doctags output only contains section_header_level_1. All nested heading levels (2–4) are flattened or lost.

Does this mean hybrid chunking for docling documents is not working here ?(Using only section_header_level_(1) for chunking?)

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### Docling version
Docling version: 2.32.0
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### Python version
Python 3.10.12

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