Context

This change addresses an issue where setSourceDataAtCell() would incorrectly update a parent row instead of the intended nested child row when the nestedRows option was enabled. The DataSource.setAtCell() method now correctly writes to the row object resolved by the modifyRowData hook, ensuring accurate updates for nested data structures.

How has this been tested?

• Unit tests: A new unit test was added to dataSource.unit.js to verify that DataSource.setAtCell() correctly writes through hook-resolved row objects, ensuring proper data layer updates for nested-like data.
• E2E tests: An E2E regression test was added to setSourceDataAtCell.spec.js to confirm that Core.setSourceDataAtCell with nestedRows: true correctly updates child data while leaving sibling parents unchanged.
• Linting: All modified files passed lint checks.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Additional language file or change to the existing one (translations)

Related issue(s):

1. The setSourceDataAtCell doesn't works on children #10657

Affected project(s):

• handsontable
• @handsontable/angular-wrapper
• @handsontable/react-wrapper
• @handsontable/vue3

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement
• My change requires a change to the documentation.