test: add an MCP App showcase config to reproduce and verify #1859

The `mcp_app_demo` tool and `mcp_app_demo_widget` UI resource presets already
existed in the preset registry, but no config wired them into a server — so
there was no runnable way to reach a rendered MCP App, and therefore no way to
exercise the sandbox proxy path that #1859 broke.

`mcp-app-http.json` composes the two over plain streamable-HTTP. With it, the
Apps tab renders a real widget and the packaging bug is reproducible on demand:
without clients/web/static in the tarball the widget area shows "Sandbox not
loaded: ENOENT … /clients/web/static/sandbox_proxy.html" instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAR2Nr9kXbrywFNUVoTe9F

test-servers/configs/mcp-app-http.json

@@ -0,0 +1,12 @@
+{
+  "serverInfo": {
+    "name": "mcp-app-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "echo" }, { "preset": "mcp_app_demo" }],
+  "resources": [{ "preset": "mcp_app_demo_widget" }],
+  "transport": {
+    "type": "streamable-http",
+    "port": 3130
+  }
+}