Relative hyperlinks are exported with backslashes on Windows (Markdown, HTML, LaTeX, DocLang, JSON)

### Bug

Relative hyperlinks are stored on `TextItem.hyperlink` as a `Path` (the field is `AnyUrl | Path`). On Windows that is a `WindowsPath`, and the serializers turn the hyperlink into text with `str(...)`, so a link written as `sub/next.html` in the source comes out as `sub\next.html`:

- Markdown: `[next page](sub\next.html)` (`MarkdownDocSerializer.serialize_hyperlink` uses `f"[{text}]({hyperlink!s})"`)
- HTML: `` (`HTMLDocSerializer.serialize_hyperlink`)
- LaTeX: `serialize_hyperlink` uses `str(hyperlink)`
- DocLang: `_text_item_hyperlink_uri` returns `str(item.hyperlink)`
- JSON: `save_as_json` stores `"sub\\next.html"`, so a document converted on Windows carries a different path than the same document converted on Linux, and loading it on Linux gives a single path component that contains a backslash.

```html

See the next page.

```python
from docling.document_converter import DocumentConverter

doc = DocumentConverter().convert("index.html").document
print(repr(doc.texts[0].hyperlink)) # WindowsPath('sub/next.html')
print(doc.export_to_markdown()) # See the [next page](sub\next.html) .
```

`export_to_html()` gives ``.

Python 3.13.12

Start with MarkdownDocSerializer.serialize_hyperlink, HTMLDocSerializer.serialize_hyperlink, LaTeX serialize_hyperlink, DocLang _text_item_hyperlink_uri, and save_as_json. Review the existing _escape_uri_path helper used for image paths and reproduce the Windows case from the issue. Done means relative hyperlinks use forward slashes consistently in Markdown, HTML, LaTeX, DocLang, and JSON, while Linux and macOS output remains unchanged.

Written by the indexing model from the issue text.