fix(ai-chat reference): await Chat.messages persist in onTurnStart

The reference's onTurnStart was using chat.defer for the messages write, which is fire-and-forget. If a user refreshed the page mid-stream, getChatMessages returned [] (the deferred write hadn't landed yet), useChat hydrated with empty initialMessages, and the resumed SSE stream pushed the assistant into an empty array — the user's message vanished from the rendered conversation forever.

Switch to await prisma.chat.update(...) so the write is durable before chat.agent begins streaming. Verified end-to-end against test cloud: mid-stream refresh now yields [user, assistant] with no duplication.

Aligns with the Warning added to docs/ai-chat/patterns/database-persistence.mdx in the docs branch.

Author: ericallam

references/ai-chat/src/trigger/chat.ts

@@ -368,12 +368,15 @@ export const aiChat = chat
     onTurnStart: async ({ chatId, uiMessages, writer, runId }) => {
       warmCodeSandbox(runId);
       writer.write({ type: "data-turn-status", data: { status: "preparing" }, transient: true });
-      chat.defer(
-        prisma.chat.update({
-          where: { id: chatId },
-          data: { messages: uiMessages as unknown as ChatMessagesForWrite },
-        })
-      );
+      // Awaited (not chat.defer) so the user message is durable before
+      // streaming begins. A mid-stream page refresh reads from DB; if the
+      // write is still in flight, getChatMessages returns [] and the
+      // resumed SSE stream rebuilds an assistant-only conversation,
+      // dropping the user message from the UI.
+      await prisma.chat.update({
+        where: { id: chatId },
+        data: { messages: uiMessages as unknown as ChatMessagesForWrite },
+      });
     },
     // #endregion