feat(pptx): preserve hyperlinks and list hierarchy in PPTX conversion

[CheenDing]

• Add _convert_text_frame_to_markdown() to extract text paragraph-by-paragraph
• Add _convert_paragraph_to_markdown() to preserve hyperlinks as text
• Handle bullet list indentation based on paragraph.level
• Escape brackets in hyperlink text to avoid breaking markdown syntax
• Add test PPT and test vector for hyperlinks and nested lists

Fixes the issue where PPTX hyperlinks were lost and bullet list hierarchy was flattened into plain text.

Summary

This PR improves PPTX-to-Markdown conver
sion by preserving hyperlinks and **
bullet list hierarchy** that were previo
usly lost.

Problem

Before this change:

• Hyperlinks in PPTX text were stripped,
  leaving only plain text
• Bullet list indentation levels were fl
  attened into a single text block

Example input (PPTX slide):
Visit Microsoft (hyperlink to https://mi
crosoft.com) • Level 1 item • Level 2 it
em

Before (incorrect):
Visit Microsoft Level 1 item Level 2 ite
m

After (correct):
Visit Microsoft
• Level 1 item
• Level 2 item