fix(DataTable): correct divider thickness in expanded rows

[kalai7m]

When a DataTable row is expanded, the divider in the expanded content appeared thicker than expected. Hovering over the expanded row also showed a double line, which affected the visual consistency of the table. This fix ensures consistent divider thickness and hover behavior for expanded rows.

Fixes: #20033

Closes #

Fixed visual inconsistencies in DataTable expanded rows

Changelog

New

• {{new thing}}

Changed

• Updated hover styles to ensure consistent single-line borders between parent and child rows
• Aligned direct parent hover behavior with programmatic hover class behavior for visual consistency

Removed

• Removed box-shadow from expanded child row cells to prevent thick divider appearance

Testing / Reviewing

1. Open a DataTable with expandable rows
2. Expand a row and verify the divider within the expanded content appears as a single, consistent line (not thicker)
3. Hover over the expanded parent row and verify no double line appears
4. Hover over the expanded child row content and verify the parent row receives the same hover styling
5. Test in both regular and zebra-striped DataTable variants
6. Verify selected row states still display correctly with expanded rows

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• Updated documentation and storybook examples
• Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

More details can be found in the pull request guide

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