fix: LS26002468: insert row - reset initialValue and persist col data

[zammarchi-smeup]

This pull request makes improvements to how cell data is managed when creating or resetting rows in the KupDataTable component. The main focus is on ensuring that both the value and the initialValue properties of cell data are correctly reset, and on preserving and extending cell data properties when constructing new cells.

Cell data management improvements:

• When resetting cell values, the code now also clears the initialValue property inside cell.data if it exists, ensuring that both the displayed value and its initial state are reset.
• When constructing new cells, the code now spreads existing properties from c.data into the new cell's data object, and conditionally adds size and maxLength properties if present, ensuring that all relevant cell data is preserved and extended as needed.