fix: correct CSV physical cell count

[skytin1004]

Purpose of the pull request

While looking at the CSV row API from a workbook write handler, I noticed that CsvRow#getPhysicalNumberOfCells() was returning the row index instead of the number of cells in the row.

I checked the implementation and found that getPhysicalNumberOfCells() was calling getRowNum(). That means the first row returns 0, even when the row actually contains cells. For example, a CSV header row with No, Name, and Age should report 3 physical cells, but it reported 0.

What's changed?

I changed CsvRow#getPhysicalNumberOfCells() to return cellList.size() instead of getRowNum().

I also added a regression test in CsvFormatTest. The test writes a CSV file with three header cells, then checks the first row from a workbook write handler and verifies that getPhysicalNumberOfCells() returns 3.

Tested with CsvFormatTest#testPhysicalNumberOfCells, full CsvFormatTest, and spotless:check.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

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[delei]

LGTM