a11y: make message history and action buttons accessible to screen readers

[drewpoling2]

• Add role=log, aria-label, and aria-live to message history container
• Add tabIndex and aria-label to individual message wrappers
• Pass aria-label through HeaderButtonWithToolTip to underlying button
• Make response action buttons keyboard accessible (tabIndex 0)

Fixes #12331

Description

The Continue chat UI was not accessible to screen reader users.
Message history had no semantic role or label, individual messages
were not in the tab order, and action buttons lacked aria-labels.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

N/A - changes are aria attributes and tab order, not visual

Tests

Manually verified with VoiceOver on macOS (Cmd+F5). Message history
container is now announced, individual messages are reachable via tab,
and action buttons are announced with descriptive labels.

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Summary by cubic

Make chat message history and response action buttons accessible to screen readers and keyboard users by adding roles, labels, and tab navigation. Fixes #12331.

• Bug Fixes
  • Set role="log", aria-label "Message history", and aria-live="polite" on the message list container.
  • Added tabIndex=0 and descriptive aria-labels to each message wrapper.
  • Passed ariaLabel through HeaderButtonWithToolTip to the underlying button.
  • Ensured response action buttons are focusable and in the tab order.

^{Written for commit 0dc46a7. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[drewpoling2]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[drewpoling2]

CI failure appears to be in the IntelliJ integration test suite which is unrelated to these changes. My changes are scoped to the VS Code GUI layer only (Chat.tsx, HeaderButtonWithToolTip.tsx, ResponseActions.tsx). Happy to investigate further if needed.

[itxaiohanglover]

Nice a11y fix! Adding aria-label with fallback to text ensures screen readers can identify buttons. The import cleanup is a tidy bonus.