fix collapsible content header focus ring cut

Author: OEvgeny

packages/fluent-theme/src/components/activity/PartGroupingDecorator.module.css

@@ -761,6 +761,11 @@
       border-style: solid;
     }
 
+    &.part-grouping-decorator--group :global(.webchat__bubble .webchat__bubble__content) {
+      margin-block: calc(var(--webchat-spacingVerticalS) * -1);
+      padding-block: var(--webchat-spacingVerticalS);
+    }
+
     &.part-grouping-decorator--from-bot:not(.part-grouping-decorator--group) :global(.webchat__bubble__content) {
       box-sizing: content-box;
       padding-block: var(--webchat-spacingVerticalM);