fix: synchronize ColumnLookup when splitting multi-column ranges (#2471)

[lievendf]

Fixes #2471

Summary

When applying style or format changes to a sub-range (e.g. B:C) inside a previously styled multi-column range (e.g. A:J), the truncated column span (e.g. D:J) updates ColumnMax in the column object, but ColumnLookup is not notified of the updated boundary key. Subsequent cell style lookups beyond column C fail TryGetExcelColumn boundary checks and incorrectly fall back to default style 0 (11pt font). Furthermore, calling GetColumn inside SetStyleInner caused recursive span fragmentation across adjacent columns.

Changes

1. Synchronize ColumnLookup: Call ColumnLookup.UpdateColumn(column.ColumnMin, column) whenever column.ColumnMax or column._columnMax is modified during range splitting in ExcelWorksheet.cs and ExcelStyles.cs.
2. Prevent Recursive Column Splitting: Replace GetColumn(col) with GetValueInner(0, col) as ExcelColumn inside SetStyleInner in ExcelWorksheet.cs.
3. Unit Test: Added unit test MultiColumnStyle_WhenSplitBySubRange_ShouldInheritStyle in ExcelStyleTests.cs.

[JanKallman]

Thanks for this PR! However, even though this fix works the core problem here is that the styleId is set before the ExcelColumn is written to the cell store. This causes the style to be written to the old ExcelColumn object. With this fix not UpdateColumn calls are needed as these are done in the SetValueInner method. I'll add credit to your in the release note for this.