Add MCP Apps hello world demo tool

Implements a simple MCP Apps tool that returns an interactive React UI:

- Add `mcp_apps_hello_world` tool with `_meta.ui/resourceUri` metadata
- Add resource handler for `ui://hello-world/app.html` returning HTML
- Create React app using `@modelcontextprotocol/ext-apps/react` hooks
- Add Vite build tooling to bundle React apps into single HTML files

The tool demonstrates the MCP Apps extension (SEP-1865) which allows
MCP servers to return interactive UI that runs in the client.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

vite.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+import { viteSingleFile } from "vite-plugin-singlefile";
+import path from "path";
+
+const input = process.env.INPUT;
+if (!input) {
+  throw new Error("INPUT environment variable is required");
+}
+
+export default defineConfig({
+  plugins: [react(), viteSingleFile()],
+  root: path.dirname(input),
+  build: {
+    outDir: path.resolve("dist/apps"),
+    emptyOutDir: false,
+    rollupOptions: {
+      input: input,
+    },
+  },
+});