test(test-servers): export MCP_APP_DEMO_URI, dedupe in integration test

Address code-review DRY note on PR #1587: MCP_APP_DEMO_URI was defined
in both the fixture and the integration test. Export it from the fixture
and import it in the test so the two stay in lockstep.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3fTN8H3R8YV4yUGvZjYnX

clients/web/src/test/integration/mcp/mcp-app-demo.test.ts

@@ -7,11 +7,10 @@ import {
   createTestServerInfo,
   createMcpAppDemoTool,
   createMcpAppDemoResource,
+  MCP_APP_DEMO_URI,
 } from "@modelcontextprotocol/inspector-test-server";
 import type { Tool } from "@modelcontextprotocol/sdk/types.js";
 
-const MCP_APP_DEMO_URI = "ui://demo/widget.html";
-
 /**
  * Exercises the `mcp_app_demo` preset added in #1557 end-to-end: it confirms the
  * `_meta` plumbing through the composable test server's tool/resource