Add comprehensive tests for services (ConnectionTestService, RuntimeConfigValidation),
ViewModels (PipelineEditor, dialogs, transformers), and Avalonia headless UI tests
for views and forms.
Add config file loading and saving service following TDD approach:
- IConfigFileService interface for loading/saving config files
- ConfigLoadException for descriptive error handling
- ConfigFileService implementation with JSON serialization
- Unit tests with mocked IFileSystem dependency