Failed to parse pdf from docling >= 2.24

Thanks for all the work! this lib is amazing!

### Bug
We have some pdfs as tests and this one started to fail after upgrading to v2.24.0

[pdf_two_col.pdf](https://github.com/user-attachments/files/18996530/pdf_two_col.pdf)

.venv/lib/python3.11/site-packages/docling/models/readingorder_model.py:345: AssertionError
AssertionError: Merged element must be of same type as element.
...

### Steps to reproduce
```
pdf_pipeline_options = PdfPipelineOptions(
artifacts_path=os.getenv("DOCLING_ARTIFACTS_PATH"),
accelerator_options=AcceleratorOptions(
device=AcceleratorDevice.AUTO,
),
ocr_options=EasyOcrOptions(
force_full_page_ocr=False,
use_gpu=True,
),
do_ocr=false,
table_structure_options=TableStructureOptions(
do_cell_matching=True, mode=TableFormerMode.FAST
),
generate_page_images=False,
generate_picture_images=False,
)
```
...

### Docling version
2.24.0 / 2.25.0
...

### Python version
3.13
...