### What problem does this PR solve?
Files /file API to RESTFul style.
### Type of change
- [x] Documentation Update
- [x] Refactoring
---------
Co-authored-by: writinwaters <cai.keith@gmail.com>
Co-authored-by: Liu An <asiro@qq.com>
### What problem does this PR solve?
Codecov’s coverage report shows that several RAGFlow code paths are
currently untested or under-tested. This makes it easier for regressions
to slip in during refactors and feature work.
This PR adds targeted automated tests to cover the files and branches
highlighted by Codecov, improving confidence in core behavior while
keeping runtime functionality unchanged.
### Type of change
- [x] Other (please describe): Test coverage improvement (adds/extends
unit and integration tests to address Codecov-reported gaps)
