Add region role for Chat History

[sushmi21]

This is a minor accessibility improvement adding a role="region" landmark to the chat log container (ChatScroller).

role="log" was considered but rejected: that role implies aria-live="polite" behaviour, causing screen readers to automatically announce every new message appended to the container. The Webchat already has a dedicated ScreenReaderLiveRegion component (src/webchat-ui/components/presentational/ScreenReaderLiveRegion.tsx) that manages live announcements with aria-live="polite". Using role="log" on the scroll container would duplicate those announcements and create a noisy, unpredictable screen reader experience. Adding aria-live="off" to suppress the implicit live behaviour of role="log" was also considered, but browser and screen reader support for overriding a role's implicit aria-live value is inconsistent — some screen readers ignore the override and announce anyway. On the other hand, role="region" gives the area a named landmark without implying any live-region behaviour.

Success criteria

• The chat log container in the Webchat UI is correctly identified as a landmark region by assistive technologies
• Screen readers announce the chat history area as a "region" landmark, labelled by the existing webchatChatHistoryHeading element
• Users navigating by landmarks (e.g. via screen reader shortcut) can jump directly to the chat log

How to test

1. Open the Webchat UI in a browser with a screen reader enabled (e.g. NVDA + Firefox, or VoiceOver + Safari)
2. Use the screen reader's landmark navigation shortcut to list or cycle through page regions
3. Verify that the chat history area appears as a named region (e.g. "Chat History, region" or similar, depending on the screen reader)
4. Confirm that the chat log content is accessible and that the region label matches the heading text tied to webchatChatHistoryHeading

Security

• Possible injection vector
• Authentication/Access controls touched
• Sensitive Data could be exposed
• XSS
• Logging/Monitoring touched
• Exchanges data with external systems
• No security implications

Additional considerations

• This PR might have performance implications

[graymalkin77]

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Pull request overview

Adds an accessibility landmark to the Webchat chat history container so assistive technologies can identify and navigate to the chat log as a named region.

Changes:

• Add role="region" to the chat log container in ChatScroller.
• Keep labeling via existing aria-labelledby="webchatChatHistoryHeading" to provide an accessible name without introducing implicit live-region behavior.