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/MarkdownTextRenderer.tsx

@@ -264,7 +264,7 @@ export default function MarkdownTextRenderer({
         if (fence) {
           return (
             <CodeBlock
-              language={fence.language}
+              language={fence.language || "text"}
               code={fence.code}
               className="my-3"
               highlight={highlightCode}