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
Add file system abstraction to enable testability for file operations.
- IFileSystem interface with common file operations
- FileSystem implementation wrapping System.IO
- Unit tests for FileExists functionality
Joseph Doherty