Strikethrough not working

### Bug
Not 100% sure if this is a bug or lack-of-support. Here's the example PDF containing a strikethrough:

[strikethrough.pdf](https://github.com/user-attachments/files/22523402/strikethrough.pdf)

### Steps to reproduce
With out of the box configuration it does not return and strikethroughs in the markdown. Also tried with a custom serializer:

```python
from docling.document_converter import DocumentConverter
from docling_core.transforms.serializer.markdown import MarkdownDocSerializer, MarkdownParams

class CustomMarkdownSerializer(MarkdownDocSerializer):
def serialize_strikethrough(self, text: str, **kwargs):
return f"{text}"

def converter():
return DocumentConverter()

doc_converter = converter()
conv_result = doc_converter.convert("./strikethrough.pdf")

# Use default markdown export first for comparison
print("=== Default ===")
print(conv_result.document.export_to_markdown())
print("\n" + "="*50 + "\n")

# Custom serializer
print("=== Custom serializer ===")
custom_serializer = CustomMarkdownSerializer(
doc=conv_result.document,
params=MarkdownParams()
)
custom_result = custom_serializer.serialize()
print(custom_result.text)
```

Though I see support at least at some level was added in https://github.com/docling-project/docling-core/pull/182.

### Docling version
```bash
2025-09-24 17:42:40,321 - INFO - Loading plugin 'docling_defaults'
2025-09-24 17:42:40,321 - INFO - Registered ocr engines: ['easyocr', 'ocrmac', 'rapidocr', 'tesserocr', 'tesseract']
Docling version: 2.54.0
Docling Core version: 2.48.2
Docling IBM Models version: 3.9.1
Docling Parse version: 4.5.0
Python: cpython-311 (3.11.11)
Platform: macOS-26.0-arm64-arm-64bit
```

### Python version
```bash
Python 3.11.11
```