fix(ui): add missing cell styles to table selection column

The selection cell in multi-select tables was missing the base cell
styles, causing border styling to be incomplete. This adds the cell
class alongside the cellSelection class to ensure proper styling.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/stupid-words-fail.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed missing border styles on table selection cells in multi-select mode.
+
+Affected components: Table

packages/ui/src/components/Table/components/Row.tsx

@@ -47,6 +47,7 @@ export function Row<T extends object>(props: RowProps<T>) {
         <ReactAriaCell
           className={clsx(
             classNames.cellSelection,
+            styles[classNames.cell],
             styles[classNames.cellSelection],
           )}
         >