feat: add MessagingProvider and useAdapter

packages/iios-messaging-ui/src/provider.test.tsx

@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { MessagingProvider, useAdapter } from './provider';
+import { MockAdapter } from './adapters/mock';
+
+function ShowActor() {
+  const adapter = useAdapter();
+  return <span>{adapter.currentActorId()}</span>;
+}
+
+describe('MessagingProvider', () => {
+  it('supplies the injected adapter to descendants', () => {
+    render(
+      <MessagingProvider adapter={new MockAdapter()}>
+        <ShowActor />
+      </MessagingProvider>,
+    );
+    expect(screen.getByText('me')).toBeDefined();
+  });
+
+  it('throws a helpful error when a hook is used outside the provider', () => {
+    // React logs the error boundary trace; that noise is expected.
+    expect(() => render(<ShowActor />)).toThrow(/useAdapter must be used within a <MessagingProvider>/);
+  });
+});