fix: resolve 48 lint errors in test files

- Remove whitespace from 22 blank lines (W293)
- Remove 6 unused imports (F401): os, Mock, MagicMock, Path
- Add missing imports for 12 test functions (F821): sanitize_filename, extract_remote_path, MagicMock
- Fix 1 unsorted import block (I001)

All ruff checks now pass successfully.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_file_splitting.py

@@ -517,9 +517,10 @@ class TestSplitPdfBySizeEdgeCases:
 
     def test_returns_empty_list_for_zero_page_pdf(self, tmp_path):
         """Test handling of PDF with zero pages."""
-        from app.utils.file_splitting import split_pdf_by_size
         from pypdf import PdfWriter
 
+        from app.utils.file_splitting import split_pdf_by_size
+
         # Create a technically valid but empty PDF
         empty_pdf = tmp_path / "empty.pdf"
         writer = PdfWriter()