fix(shell-docs): replace ... placeholder + add missing handler in a2a route.ts snippet

Section 3 "Configure the A2A middleware" had a literal ... ellipsis where
the imports should go, and the code block ended at runtime definition
with no POST handler — pasted code would not compile. Add the imports
the block actually uses (HttpAgent, A2AMiddlewareAgent, CopilotRuntime,
ExperimentalEmptyAdapter, copilotRuntimeNextJSAppRouterEndpoint,
NextRequest) and the canonical Next.js App Router POST handler that
hands requests to the runtime. Pattern matches the ms-agent-framework
quickstart route.ts.

Author: samjulien

showcase/shell-docs/src/content/docs/agentic-protocols/a2a.mdx

@@ -74,8 +74,14 @@ To use A2A agents with CopilotKit, you need to first install the CopilotKit midd
   <Step>
     ### Configure the A2A middleware
     ```tsx title="app/api/copilotkit/route.ts"
-
-    ...
+    import {
+      CopilotRuntime,
+      ExperimentalEmptyAdapter,
+      copilotRuntimeNextJSAppRouterEndpoint,
+    } from "@copilotkit/runtime";
+    import { HttpAgent } from "@ag-ui/client";
+    import { A2AMiddlewareAgent } from "@ag-ui/a2a-middleware";
+    import { NextRequest } from "next/server";
 
     // These first two are the urls to the a2a agents
     const researchAgentUrl = process.env.RESEARCH_AGENT_URL || "http://localhost:9001";
@@ -140,7 +146,17 @@ To use A2A agents with CopilotKit, you need to first install the CopilotKit midd
       },
     });
 
+    // Standard Next.js App Router handler that hands requests to the runtime.
+    const serviceAdapter = new ExperimentalEmptyAdapter();
 
+    export const POST = async (req: NextRequest) => {
+      const { handleRequest } = copilotRuntimeNextJSAppRouterEndpoint({
+        runtime,
+        serviceAdapter,
+        endpoint: "/api/copilotkit",
+      });
+      return handleRequest(req);
+    };
     ```
 
   </Step>