Fix JSR publishing hang for @fedify/testing

Fixed JSR publishing hanging indefinitely at the processing stage by
hiding complex type exports from the public API. The JSR type analyzer
struggled with complex type dependencies when analyzing MockFederation,
TestFederation, TestContext, and SentActivity types.

Changes:

- MockFederation class is no longer exported from public API
- TestFederation, TestContext, SentActivity interfaces are not exported
- Updated all tests to use createFederation() factory function
- Updated documentation to remove type imports and use type inference
- Types are still available through TypeScript type inference

This approach minimizes the public API surface that JSR needs to analyze
while maintaining full type safety for users.

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



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