Implement message list virtualization

[liangfung]

Summary

Implement virtualization for the MessageList component to improve performance in long conversations.

Details

Currently, MessageList renders all messages in the conversation directly within a ScrollArea. As the number of messages grows, this can lead to performance degradation. Implementing virtualization will ensure that only the visible messages are rendered.

Target component: packages/vscode-webui/src/components/message/message-list.tsx

This is a sub-issue of #1488.

Proposed Changes

• Integrate a virtualization library (e.g., react-virtuoso or react-window).
• Refactor MessageList to use the virtualization component.
• Ensure that scroll-to-bottom and other existing functionalities remain intact.

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