7 tests covering generateTemplate(), quizToXlsx(), and xlsxToQuiz():
- valid XLSX output and MIME type
- template without example reimports as empty
- template example data survives a reimport
- round-trip (export → reimport) preserves questions, answers, and correct flags
- empty quiz exports and reimports cleanly
- invalid MIME type throws InvalidArgumentException
- question with no answers throws SpreadsheetDataException with error list
