When fenced code blocks have no language specifier, react-syntax-highlighter
receives undefined for the language prop, causing a white screen crash.
- CodeBlock.tsx: fallback to 'text' when language is undefined
- MarkdownTextRenderer.tsx: defensive fallback at fence rendering site
- Added test cases for both components
Fixes#4116
Batch stream deltas, window long transcripts, lazy-load syntax highlighting, and refine activity/composer interactions.
Add title refresh retries plus tests for streaming, windowing, code blocks, and live activity behavior.