### What problem does this PR solve?
Fix return type annotation in truncate function
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._
### Type of change
- [x] Refactoring
Signed-off-by: Jin Hai <haijin.chn@gmail.com>
### What problem does this PR solve?
_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._
Issue link:#[[Link the issue
here](https://github.com/infiniflow/ragflow/issues/200)]
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
