GitLab

- Replace MuPDF with pdfminer for detecting invalid characters in PDFs
- Uncomment and update the detect_invalid_chars function to use pdfminer
- Update the check_invalid_chars function in pdf_meta_scan.py to use the new implementation

- Replace pdfminer with PyMuPDF for character detection
- Implement new method detect_invalid_chars_by_pymupdf
- Update check_invalid_chars in pdf_meta_scan.py to use new method
- Add __replace_0xfffd function in pdf_parse_union_core_v2.py to handle special characters
- Remove unused imports and update requirements.txt