Fix HSSFSheet.ShiftRows: PhysicalNumberOfRows not decreasing when shifting rows up

[Copilot]

HSSFSheet.ShiftRows with negative n correctly decremented LastRowNum but left vacated row objects in the internal rows dictionary, so PhysicalNumberOfRows stayed stale. XSSF handled this correctly; HSSF did not.

sheet.ShiftRows(3, sheet.LastRowNum, -1);
// Before: LastRowNum=3, PhysicalNumberOfRows=5  ← bug
// After:  LastRowNum=3, PhysicalNumberOfRows=4  ← correct

Changes

• HSSFSheet.ShiftRows — After the shift loop, call RemoveRow() on each vacated position in the range [Math.Max(startRow, endRow + n + 1), endRow]. These rows had their cells moved out but persisted as empty objects in the rows dictionary, inflating PhysicalNumberOfRows.

• BaseTestSheetShiftRows — Added TestShiftRowsUpPhysicalRowCount asserting both LastRowNum and PhysicalNumberOfRows decrease after an upward shift. Runs for HSSF and XSSF.

• TestBugs.Test50416LastRowNumber — Updated assertions that previously expected vacated rows to survive as empty objects (GetRow(n).LastCellNum == -1). With the fix, those positions are null; updated to IsNull and removed the now-unreachable RemoveRow call.

Original prompt

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This section details on the original issue you should resolve

<issue_title>When moving a row up in HSSFWorkbook, the LastRowNum decreases, but the physical rows do not decrease.</issue_title>
<issue_description>### NPOI Version
2.7.1

File Type

• XLS

Upload the Excel File

input.xls

Reproduce Steps

string inputFile = "input.xls";
IWorkbook workbook;

using (FileStream file = new FileStream(inputFile, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
    workbook = new HSSFWorkbook(file);
}


ISheet sheet = workbook.GetSheetAt(0);

Console.WriteLine(sheet.LastRowNum);
Console.WriteLine(sheet.PhysicalNumberOfRows);

sheet.ShiftRows(3, sheet.LastRowNum, -1);

Console.WriteLine(sheet.LastRowNum);
Console.WriteLine(sheet.PhysicalNumberOfRows);

string outputFile = "output.xls";
using (FileStream file = new FileStream(outputFile, FileMode.Create, FileAccess.Write))
{
    workbook.Write(file);
}

Issue Description

As shown in the code above, after I moved all rows below the third row up by one, the sheet.LastRowNum decreased by 1, but the sheet.PhysicalNumberOfRows remained unchanged.
Unlike XSSFWorkbook, where both sheet.LastRowNum and sheet.PhysicalNumberOfRows decrease by 1.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes When moving a row up in HSSFWorkbook, the LastRowNum decreases, but the physical rows do not decrease. #1424

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