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.

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

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

---------

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

src/app-bridge.ts

@@ -34,6 +34,7 @@ import {
 import {
   type McpUiSandboxResourceReadyNotification,
   type McpUiSizeChangedNotification,
+  type McpUiToolCancelledNotification,
   type McpUiToolInputNotification,
   type McpUiToolInputPartialNotification,
   type McpUiToolResultNotification,
@@ -737,6 +738,43 @@ export class AppBridge extends Protocol<Request, Notification, Result> {
     });
   }
 
+  /**
+   * Notify the Guest UI that tool execution was cancelled.
+   *
+   * The host MUST send this notification if tool execution was cancelled for any
+   * reason, including user action, sampling error, classifier intervention, or
+   * any other interruption. This allows the Guest UI to update its state and
+   * display appropriate feedback to the user.
+   *
+   * @param params - Optional cancellation details:
+   *   - `reason`: Human-readable explanation for why the tool was cancelled
+   *
+   * @example User-initiated cancellation
+   * ```typescript
+   * // User clicked "Cancel" button
+   * bridge.sendToolCancelled({ reason: "User cancelled the operation" });
+   * ```
+   *
+   * @example System-level cancellation
+   * ```typescript
+   * // Sampling error or timeout
+   * bridge.sendToolCancelled({ reason: "Request timeout after 30 seconds" });
+   *
+   * // Classifier intervention
+   * bridge.sendToolCancelled({ reason: "Content policy violation detected" });
+   * ```
+   *
+   * @see {@link McpUiToolCancelledNotification} for the notification type
+   * @see {@link sendToolResult} for sending successful results
+   * @see {@link sendToolInput} for sending tool arguments
+   */
+  sendToolCancelled(params: McpUiToolCancelledNotification["params"]) {
+    return this.notification(<McpUiToolCancelledNotification>{
+      method: "ui/notifications/tool-cancelled",
+      params,
+    });
+  }
+
   /**
    * Send HTML resource to the sandbox proxy for secure loading.
    *