docs(showcase/ag2): add headless-complete::page-send-message region

Wraps the send-handler block (threadId/useAgent/useCopilotKit + connect
effect + handleSubmit + handleStop) in src/app/demos/headless-complete/page.tsx
with @region[page-send-message]. Matches the canonical langgraph-python
span.

Author: samjulien

showcase/integrations/ag2/src/app/demos/headless-complete/page.tsx

@@ -68,6 +68,7 @@ export default function HeadlessCompleteDemo() {
 // the agent, wires up the connect/run/stop lifecycle, and hands the pure
 // presentational pieces their props.
 function Chat() {
+  // @region[page-send-message]
   const threadId = useMemo(() => crypto.randomUUID(), []);
   const { agent } = useAgent({ agentId: AGENT_ID, threadId });
   const { copilotkit } = useCopilotKit();
@@ -122,6 +123,7 @@ function Chat() {
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [agent]);
+  // @endregion[page-send-message]
 
   // Wrap the chat body in a CopilotChatConfigurationProvider so the
   // rendering primitives used inside `useRenderedMessages` see a