### What problem does this PR solve?
fix #6085 
RagTokenizer's dfs_() function falls into infinite recursion when
processing text with repetitive Chinese characters (e.g.,
"一一一一一十一十一十一..." or "一一一一一一十十十十十十十二十二十二..."), causing memory leaks.
### Type of change
Implemented three optimizations to the dfs_() function:
1.Added memoization with _memo dictionary to cache computed results
2.Added recursion depth limiting with _depth parameter (max 10 levels)
3.Implemented special handling for repetitive character sequences
- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>