fix: ensure newlines around think tags for proper markdown rendering (#20594)

web/app/components/base/markdown/markdown-utils.ts

@@ -33,5 +33,6 @@ export const preprocessThinkTag = (content: string) => {
   return flow([
     (str: string) => str.replace(thinkOpenTagRegex, '<details data-think=true>\n'),
     (str: string) => str.replace(thinkCloseTagRegex, '\n[ENDTHINKFLAG]</details>'),
+    (str: string) => str.replace(/(<\/details>)(?![^\S\r\n]*[\r\n])(?![^\S\r\n]*$)/g, '$1\n'),
   ])(content)
 }