fix: prevent infinite loop in extract_toc_content

[luojiyin1987]

Fixes #64

Summary

Fix broken infinite loop protection in extract_toc_content().

Problem

The function used len(chat_history) > 5 to exit the loop, but chat_history was rebuilt every iteration with exactly 2 elements, making the check never true.

Change

• Add explicit attempt counter initialized to 0
• Add max_attempts = 5 constant
• Check attempt > max_attempts instead of len(chat_history) > 5
• Move check to beginning of loop for earlier exit

Test

Manual code review - the logic now correctly counts iterations and exits after 5 attempts.