fix: Use tabIndex={-1} to avoid breaking focus traps in Adaptive Cards

tabIndex={0} added an extra tab stop inside the card's focus trap,
causing SHIFT-TAB from card controls to land on the attachment row
instead of the expected element. Using tabIndex={-1} makes the row
programmatically focusable (for screen readers via virtual cursor)
without adding it to the Tab order.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: uzirthapa

__tests__/html2/accessibility/adaptiveCard/attachmentRow.focusable.html

@@ -58,10 +58,10 @@
 
         const attachmentRows = document.querySelectorAll('[aria-roledescription="attachment"]');
 
-        // Both attachment rows should be focusable via tabIndex.
+        // Both attachment rows should be programmatically focusable via tabIndex="-1".
         expect(attachmentRows).toHaveProperty('length', 2);
-        expect(attachmentRows[0].getAttribute('tabindex')).toBe('0');
-        expect(attachmentRows[1].getAttribute('tabindex')).toBe('0');
+        expect(attachmentRows[0].getAttribute('tabindex')).toBe('-1');
+        expect(attachmentRows[1].getAttribute('tabindex')).toBe('-1');
 
         // Both should have role="group".
         expect(attachmentRows[0].getAttribute('role')).toBe('group');

packages/component/src/Activity/AttachmentRow.tsx

@@ -39,7 +39,7 @@ function AttachmentRow(props: AttachmentRowProps) {
       aria-roledescription="attachment"
       className={classNames['stacked-layout__attachment-row']}
       role="group"
-      tabIndex={0}
+      tabIndex={-1}
     >
       <ScreenReaderText text={attachedAlt} />
       {showBubble ? (