fix(frontend): fix Mermaid preview failure in historical messages (#3196)

* fix(frontend): render historical mermaid diagrams

* fix(frontend): address mermaid review feedback

* Stabilize cancel lifecycle test

* fix(frontend): handle mermaid fence variants

* fix(frontend): normalize mermaid arrow spacing

* fix(frontend): handle mermaid CRLF fences

* chore: keep mermaid fix frontend-scoped

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/components/workspace/messages/markdown-content.tsx

@@ -7,7 +7,10 @@ import {
   MessageResponse,
   type MessageResponseProps,
 } from "@/components/ai-elements/message";
-import { streamdownPlugins } from "@/core/streamdown";
+import {
+  preprocessStreamdownMarkdown,
+  streamdownPlugins,
+} from "@/core/streamdown";
 import { cn } from "@/lib/utils";
 
 import { CitationLink } from "../citations/citation-link";
@@ -33,6 +36,10 @@ export function MarkdownContent({
   remarkPlugins = streamdownPlugins.remarkPlugins,
   components: componentsFromProps,
 }: MarkdownContentProps) {
+  const normalizedContent = useMemo(
+    () => preprocessStreamdownMarkdown(content),
+    [content],
+  );
   const components = useMemo(() => {
     return {
       a: (props: AnchorHTMLAttributes<HTMLAnchorElement>) => {
@@ -70,7 +77,7 @@ export function MarkdownContent({
       rehypePlugins={rehypePlugins}
       components={components}
     >
-      {content}
+      {normalizedContent}
     </MessageResponse>
   );
 }