fix(grid): Update grid cell active state selector specificity

Adjusted the selectors for active grid cells to ensure proper styling by increasing specificity. This fixes potential issues with nested elements inheriting unintended styles.

Author: desig9stein

projects/igniteui-angular/src/lib/core/styles/components/grid/_grid-theme.scss

@@ -1791,8 +1791,8 @@
     %grid-cell--active {
         box-shadow: inset 0 0 0 rem(1px) var-get($theme, 'cell-active-border-color');
 
-        %igx-grid__filtering-cell,
-        %grid-cell-header {
+        > %igx-grid__filtering-cell,
+        > %grid-cell-header {
             border-inline-end-color: var-get($theme, 'cell-active-border-color');
             border-bottom-color: var-get($theme, 'cell-active-border-color');
         }