fix(reasoning): prevent LLM-hallucinated HTML tags from rendering as DOM elements (#2321)

* fix

* add test

* fix

frontend/src/components/ai-elements/reasoning.tsx

@@ -11,6 +11,7 @@ import { BrainIcon, ChevronDownIcon } from "lucide-react";
 import type { ComponentProps, ReactNode } from "react";
 import { createContext, memo, useContext, useEffect, useState } from "react";
 import { Streamdown } from "streamdown";
+import { reasoningPlugins } from "@/core/streamdown/plugins";
 import { Shimmer } from "./shimmer";
 
 type ReasoningContextValue = {
@@ -177,7 +178,7 @@ export const ReasoningContent = memo(
       )}
       {...props}
     >
-      <Streamdown {...props}>{children}</Streamdown>
+      <Streamdown {...reasoningPlugins}>{children}</Streamdown>
     </CollapsibleContent>
   ),
 );