feat(snackbar): Add 1px transparent border for high contrast support

PiperOrigin-RevId: 355295203

Author: EstebanG23

packages/mdc-snackbar/_mixins.scss

@@ -26,6 +26,7 @@
 @use '@material/icon-button/mixins' as icon-button-mixins;
 @use '@material/ripple/ripple-theme';
 @use '@material/rtl/mixins' as rtl-mixins;
+@use '@material/dom/mixins' as dom-mixins;
 @use '@material/typography/typography';
 @use '@material/elevation/mixins' as elevation-mixins;
 @use '@material/shape/mixins' as shape-mixins;
@@ -97,6 +98,11 @@
   }
 
   .mdc-snackbar__surface {
+    // 1px border around the snackbar surface for high contrast mode.
+    &::before {
+      @include dom-mixins.transparent-border($query: $query);
+    }
+
     @include feature-targeting.targets($feat-structure) {
       @include rtl-mixins.reflexive-property(padding, 0, variables.$padding);