Fix type check for 'thinking' in message content (#2964)

* Fix type check for 'thinking' in message content

When Gemini via Vertex AI returns content as a string inside an array, the in operator throws TypeError because it can't be used on primitives.

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Zil6n <136249885+Zil6n@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

frontend/src/core/messages/utils.ts

@@ -251,7 +251,7 @@ export function extractReasoningContentFromMessage(message: Message) {
   }
   if (Array.isArray(message.content)) {
     const part = message.content[0];
-    if (part && "thinking" in part) {
+    if (part && typeof part === "object" && "thinking" in part) {
       return part.thinking as string;
     }
   }