demo: fix useChat demo regression from id → chatId rename

Commit 6ef8a77 renamed `SendMessagesRequestContext.id` to `chatId`
and updated the SDK source, tests, and internal docs, but missed
the useChat demo's server route, which still reads `id` from the
POST body. That left `ably.channels.get(undefined)` on the server,
so publishes targeted a fallback channel name that did not match
the `ai` namespace rule — manifesting as error 93002 ("Can only
update/delete/append messages on channels with mutableMessages
enabled") once the transport attempted any UPDATE/DELETE action.

Update the server route to read `chatId` from the body so it
attaches to the channel name the client intended.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lawrence-forooghian

demo/vercel/react/use-chat/src/app/api/chat/route.ts

@@ -21,7 +21,7 @@ interface ChatRequestBody {
   clientId: string;
   messages: MessageNode<UIMessage>[];
   history?: MessageNode<UIMessage>[];
-  id: string;
+  chatId: string;
   forkOf?: string;
   parent?: string;
 }
@@ -30,8 +30,8 @@ interface ChatRequestBody {
 const ably = new Ably.Realtime({ key: process.env.ABLY_API_KEY! });
 
 export async function POST(req: Request) {
-  const { messages, history, id, turnId, clientId, forkOf, parent } = (await req.json()) as ChatRequestBody;
-  const channel = ably.channels.get(id);
+  const { messages, history, chatId, turnId, clientId, forkOf, parent } = (await req.json()) as ChatRequestBody;
+  const channel = ably.channels.get(chatId);
 
   const transport = createServerTransport({ channel });
   const turn = transport.newTurn({ turnId, clientId, parent, forkOf, signal: req.signal });