[Fix Accessibility] Add focus-visible styles to Clipboard Copy button (#1716)

### Add focus-visible styles to clipboard copy button for improved
accessibility

- **Files Changed**
code.js

- **Summary**
This change adds proper focus-visible styling to the clipboard copy
button, providing a clear visual indicator when users navigate to the
button using keyboard controls while maintaining system-consistent focus
ring colors for better accessibility compliance.

## BEFORE
<img width="878" height="228" alt="image"
src="https://github.com/user-attachments/assets/a7c21e39-9003-4f63-9df7-071e17b1d763"
/>

## AFTER
<img width="851" height="232" alt="image"
src="https://github.com/user-attachments/assets/e59dede0-4dee-4d19-875b-60af30d3dac6"
/>

Author: jpg619

src/mdx/code.js

@@ -31,6 +31,14 @@ const ClipboardCopy = ({value, ...props}) => {
         setCopied(true)
         announce(`Copied to clipboard`)
       }}
+      sx={{
+        ...props.sx,
+        '&:focus-visible': {
+          outline: '2px solid',
+          outlineColor: '-webkit-focus-ring-color',
+          outlineOffset: '2px',
+        },
+      }}
     >
       <Octicon icon={copied ? CheckIcon : CopyIcon} sx={{color: copied ? 'success.fg' : 'fg.muted'}} />
     </Button>