fix(tests): Add Excel test fixture files to repository

The .gitignore was blocking *.xlsx files, but test fixtures need to be
tracked. Force-added the following test fixtures:
- tests/fixtures/sample_pi_directory.xlsx
- tests/fixtures/ilab_equipment_sample.xlsx
- tests/fixtures/ilab_pi_directory_sample.xlsx
- tests/fixtures/ilab_services_sample.xlsx

This fixes 3 failing tests in CI:
- TestExcelImport::test_import_excel_with_header
- TestExcelImport::test_import_excel_auto_detect
- TestDataValidation::test_row_count_accuracy

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: patchmemory