refactor(tests): Organize Excel tests into tests/excel/ subdirectory

Move Excel-related test files to dedicated subdirectory:
- test_excel_range_calculator.py -> tests/excel/
- test_excel_merged_cell_handler.py -> tests/excel/
- test_excel_pane_manager.py -> tests/excel/
- test_excel_style_extractor.py -> tests/excel/

Benefits:
- Test structure mirrors source code structure (src/excel/ <-> tests/excel/)
- Better organization and discoverability
- All 142 tests continue to pass (76 unit + 66 integration)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: k-ibaraki