feat: implement ui/notifications/tool-cancelled + add missing App.onteardown

[ochafik]

Summary

• Implement ui/notifications/tool-cancelled throughout the SDK - hosts MUST send this when tool execution is cancelled (user action, timeout, classifier intervention, etc.)
• Add App.onteardown handler for guest UIs to handle graceful shutdown requests

Changes

tool-cancelled notification

Component	File	Change
Type	src/spec.types.ts	Added McpUiToolCancelledNotification interface
Schema	src/generated/schema.ts	Auto-generated via ts-to-zod
Host method	src/app-bridge.ts	Added sendToolCancelled()
Guest handler	src/app.ts	Added ontoolcancelled setter
Exports	src/types.ts	Export type and schema
Tests	src/app-bridge.test.ts	2 tests (with/without reason)

onteardown handler

Component	File	Change
Guest handler	src/app.ts	Added onteardown setter
Capability	src/app.ts	Allow ui/resource-teardown in request handler
Tests	src/app-bridge.test.ts	2 tests (sync/async cleanup)

Usage

tool-cancelled (Host → Guest):

// Host
bridge.sendToolCancelled({ reason: "User cancelled" });

// Guest
app.ontoolcancelled = (params) => {
  console.log("Cancelled:", params.reason);
};

teardown (Host → Guest):

// Host (already existed)
await bridge.sendResourceTeardown({});

// Guest (NEW)
app.onteardown = async () => {
  await saveState();
  return {};
};

Test plan

• All 19 tests pass
• Build succeeds
• Examples build successfully

🤖 Generated with Claude Code

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commit: 1076b7d

[idosal]

@ochafik it's worth noting here that this doesn't actually wait for the UI to terminate gracefully before unmounting, which is against against the spec's recommendation (Host SHOULD wait for a response before tearing down the resource (to prevent data loss).)

[ochafik]

Good catch! Fixed in bf3b1b3 - the host now waits for the guest's teardown response before unmounting, per the spec recommendation.

Changes:

• Added close button (×) to tool call panels
• Two-phase unmount: clicking close sets isDestroying state which dims the panel and disables pointer events
• sendResourceTeardown() completes (or errors) before the component is removed
• Also handles edge case where user closes before bridge is ready

Demo apps now also have onteardown handlers with simulated 500ms cleanup to demonstrate the flow.