Fix JSR publishing hang for @fedify/testing

Fixed JSR publishing hanging indefinitely at the processing stage by
refactoring the public API to use TypeScript utility types instead of
direct class exports.

- Added TestContext<TContextData> interface using Omit/Pick to extend
  Context with testing utilities while ensuring automatic inheritance
- Added TestFederation<TContextData> interface to extend Federation
- Added createFederation() factory function for creating test instances
- Exported SentActivity interface for tracking sent activities
- Updated documentation to use createFederation() instead of MockFederation

Breaking change: MockFederation class is no longer exported. Use
createFederation() instead, which returns a TestFederation instance.

This approach ensures that when new methods are added to Context or
Federation, they are automatically included in the test interfaces
without manual updates, while maintaining JSR compatibility.

https://github.com/fedify-dev/fedify/issues/468



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