feat: implement ui/notifications/tool-cancelled + add missing App.onteardown (#120)

* feat: implement ui/notifications/tool-cancelled throughout SDK

Wire the tool-cancelled notification that hosts MUST send when tool
execution is cancelled (user action, timeout, classifier intervention, etc.):

- Add McpUiToolCancelledNotification type in spec.types.ts
- Generate Zod schema via ts-to-zod
- Add AppBridge.sendToolCancelled() method for hosts
- Add App.ontoolcancelled setter for guest UIs
- Export type and schema from types.ts
- Add tests for notification with/without reason

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

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

* feat: add App.onteardown handler for graceful shutdown

Wire the guest UI side of ui/resource-teardown request:

- Add onteardown setter in App class for guest UIs to handle teardown
- Allow ui/resource-teardown in assertRequestHandlerCapability
- Import McpUiResourceTeardownRequest/Result types
- Add tests for sync and async cleanup handlers

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

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

* feat(example): wire tool-cancelled and teardown in basic-host

Update basic-host example to properly use the new notifications:

- Send sendToolCancelled when tool call promise rejects
- Send sendResourceTeardown when React component unmounts
- Store appBridge ref for cleanup access

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

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

* fix(example): implement graceful teardown per spec recommendation

Address review feedback: the host now waits for guest's teardown
response before unmounting, following the spec: "Host SHOULD wait
for a response before tearing down the resource (to prevent data loss)."

Changes:
- Add close button to tool call panels
- Two-phase unmount: isDestroying state dims panel and disables events
- sendResourceTeardown() completes before component removal
- Non-app tool results close immediately

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

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

* feat(examples): add onteardown handlers to demo apps

Demonstrate graceful shutdown with simulated 500ms cleanup work.

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

examples/basic-server-react/src/mcp-app.tsx

@@ -1,7 +1,7 @@
 /**
  * @file App that demonstrates a few features using MCP Apps SDK + React.
  */
-import type { App } from "@modelcontextprotocol/ext-apps";
+import type { App, McpUiResourceTeardownResult } from "@modelcontextprotocol/ext-apps";
 import { useApp } from "@modelcontextprotocol/ext-apps/react";
 import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
 import { StrictMode, useCallback, useEffect, useState } from "react";
@@ -35,6 +35,12 @@ function GetTimeApp() {
     appInfo: IMPLEMENTATION,
     capabilities: {},
     onAppCreated: (app) => {
+      app.onteardown = async () => {
+        log.info("App is being torn down");
+        await new Promise((resolve) => setTimeout(resolve, 500)); // Simulate cleanup work
+        log.info("App teardown complete");
+        return {};
+      };
       app.ontoolinput = async (input) => {
         log.info("Received tool call input:", input);
       };