fix(webui): handle undefined language in code blocks

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

webui/src/components/CodeBlock.tsx

@@ -33,7 +33,7 @@ const LazyHighlightedCode = lazy(async () => {
     default({ language, code, isDark }: HighlightedCodeProps) {
       return (
         <SyntaxHighlighter
-          language={language}
+          language={language || "text"}
           style={isDark ? oneDark : oneLight}
           customStyle={{
             margin: 0,