fix - stale code block height during incremental chat layout

[xinyi-gong]

Summary

• Add a computeSize(...) override for SourceViewerComposite so code block height is measured synchronously from the current StyledText content.
• Reuse a shared height helper in both measurement and async bounds refresh to keep scrollbar-height handling consistent.
• Prevent incremental chat layout from caching stale code block heights while large code blocks are still waiting for async setBounds(...).

[Copilot]

Pull request overview

This PR improves chat code-block layout stability by ensuring SourceViewerComposite reports an up-to-date height synchronously (via computeSize(...)), reducing the chance that incremental chat layout caches stale code-block heights while async setBounds(...) is pending.

Changes:

• Override computeSize(...) in SourceViewerComposite to measure height directly from the current StyledText content.
• Factor out a shared getSourceViewerHeight(...) helper and reuse it in refreshScrollerLayout() for consistent scrollbar-height handling.