fix(webapp): wrap playground 'save' JSON.parse to return 400 on malformed input

Mirrors the 'start' case (lines 104-108) — uncaught JSON.parse on a
malformed messages form field surfaced as an unhandled 500 instead of
a clean 400. Addresses Devin review on PR #3173.

Author: ericallam

apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.action.tsx

@@ -239,7 +239,12 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
         return json({ error: "chatId is required" }, { status: 400 });
       }
 
-      const messagesData = messagesStr ? JSON.parse(messagesStr) : undefined;
+      let messagesData: unknown;
+      try {
+        messagesData = messagesStr ? JSON.parse(messagesStr) : undefined;
+      } catch {
+        return json({ error: "Invalid messages JSON" }, { status: 400 });
+      }
 
       // Extract title from the first user message if the conversation still has the default title.
       // This handles the case where a preloaded conversation gets its first real message