test(client-ky): add comprehensive test coverage
Add test suite for ky client plugin with comprehensive coverage:
- buildUrl functionality with various path/query scenarios
- zero-length body handling for all content types
- unserialized and serialized request body handling
- request, response, and error interceptors
- error handling with throwOnError option
- retry configuration
- responseStyle configuration
Tests follow patterns from client-fetch and cover all core functionality.