fix: excel header gaps and formula evaluation

[rivon0507]

This pull request improves the Excel file import functionality by enhancing formula evaluation and handling of blank headers, and updates the related tests and test resources to support these features. The main changes are grouped into improvements to Excel decoding logic and enhancements to test coverage and maintainability.

Excel decoding logic improvements:

• The ExcelCodec class now uses WorkbookFactory.create to support more Excel formats and properly closes resources, and it evaluates cell formulas when reading data. Columns with blank headers are now skipped, preventing them from being included in the parsed data. [1] [2]

Test coverage and maintainability:

• The import controller test suite adds a new test to verify that formulas are evaluated and columns with blank headers are skipped during Excel import.
• The test utility method for loading Excel metadata files is refactored to load the appropriate JSON resource based on the Excel filename, improving maintainability and flexibility for future tests. [1] [2]
• New test metadata files (__level_room_teacher_teaching_unit_group.xlsx.json and __prof.xlsx.json) are added to support the updated and new tests. [1] [2]