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/core/streamdown/preprocess.ts

@@ -0,0 +1,11 @@
+import { normalizeMermaidMarkdown } from "./mermaid";
+
+const MERMAID_BLOCK_HINT_RE = /mermaid/i;
+
+export function preprocessStreamdownMarkdown(markdown: string): string {
+  if (!MERMAID_BLOCK_HINT_RE.test(markdown) || !markdown.includes("-.->")) {
+    return markdown;
+  }
+
+  return normalizeMermaidMarkdown(markdown);
+}