fix: mark CsvCell Calendar value as a date to avoid silent empty write

[Aias00]

Summary

Fixes #973.

CsvCell.setCellValueImpl(Calendar) set cellType = NUMERIC but did not set numericCellType = NumericCellTypeEnum.DATE, so CsvSheet.buildCellValue took the number branch, found numberValue was null, and wrote an empty field — silently dropping the Calendar value. The sibling Date and LocalDateTime setters already set the date type.

Root cause

CsvCell:

@Override
protected void setCellValueImpl(Calendar value) {
    if (value == null) {
        return;
    }
    this.dateValue = LocalDateTime.ofInstant(value.toInstant(), ZoneId.systemDefault());
    this.cellType = CellType.NUMERIC;
    // missing: this.numericCellType = NumericCellTypeEnum.DATE;
}

CsvSheet.buildCellValue dispatches on csvCell.getNumericCellType() == NumericCellTypeEnum.DATE; without it the cell falls through to the number branch, where numberValue is null, and returns null → empty field.

Fix

this.numericCellType = NumericCellTypeEnum.DATE;

Verification

Added CsvRowTest.csvWrite_withCalendar_producesCorrectFile: writes a Calendar (2024-01-15 12:30:45) to a CsvCell via the POI Cell API and reads the CSV file back. Before the fix the cell was an empty field; after the fix the line contains 2024-01-15. Full fesod-sheet suite: Tests run: 668, Failures: 0.

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