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

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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

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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

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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

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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>

src/spec.types.ts

@@ -167,6 +167,19 @@ export interface McpUiToolResultNotification {
   params: CallToolResult;
 }
 
+/**
+ * @description Notification that tool execution was cancelled (Host -> Guest UI).
+ * Host MUST send this if tool execution was cancelled for any reason (user action,
+ * sampling error, classifier intervention, etc.).
+ */
+export interface McpUiToolCancelledNotification {
+  method: "ui/notifications/tool-cancelled";
+  params: {
+    /** @description Optional reason for the cancellation (e.g., "user action", "timeout"). */
+    reason?: string;
+  };
+}
+
 /**
  * @description Rich context about the host environment provided to Guest UIs.
  */