fix: improve accessibility for code block control buttons

[aradhyacp]

Description

Improves accessibility for built-in code block controls by adding explicit accessible labels and screen reader feedback for copy actions.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Fixes #556

Changes Made

• Added aria-label to CodeBlockCopyButton and CodeBlockDownloadButton
• Added a screen-reader-only status announcement for successful copy actions
• Reused existing translation keys for localized accessibility labels and messages

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

Manually verified that:

• Code block copy and download buttons expose accessible labels
• Copy action announces success state to screen readers using a status region

Screenshots/Demos

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

Added accessibility improvements without changing existing button behavior or UI appearance.

[vercel]

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