Feat: add support for multi-column PDF parsing (#10475)

### What problem does this PR solve?

Add support for multi-columns PDF parsing. #9878, #9919.

Two-column sample:
<img width="1885" height="1020" alt="image"
src="https://github.com/user-attachments/assets/0270c028-2db8-4ca6-a4b7-cd5830882d28"
/>

Three-column sample: 
<img width="1881" height="992" alt="image"
src="https://github.com/user-attachments/assets/9ee88844-d5b1-4927-9e4e-3bd810d6e03a"
/>

Single-column sample:
<img width="1883" height="1042" alt="image"
src="https://github.com/user-attachments/assets/e93d3d18-43c3-4067-b5fa-e454ed0ab093"
/>



### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

deepdoc/parser/markdown_parser.py

@@ -17,7 +17,6 @@
 
 import re
 
-import mistune
 from markdown import markdown
 
 
@@ -117,8 +116,6 @@ class MarkdownElementExtractor:
     def __init__(self, markdown_content):
         self.markdown_content = markdown_content
         self.lines = markdown_content.split("\n")
-        self.ast_parser = mistune.create_markdown(renderer="ast")
-        self.ast_nodes = self.ast_parser(markdown_content)
 
     def extract_elements(self):
         """Extract individual elements (headers, code blocks, lists, etc.)"""