https://winscp.net/tracker/856
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@mjkent: Add interfaces to .NET assembly for improved testability
This PR extracts interfaces from the main WinSCP .NET assembly classes (Session, SessionOptions, TransferOptions, etc.), enabling:
Unit testing - Mock ISession to test code that uses WinSCP without requiring actual SFTP/FTP connections
Dependency injection - Register interfaces in DI containers for cleaner architecture
Decoupling - Program against abstractions rather than concrete implementations
This change should be non-breaking, but I'm new to this project so review and feedback appreciated!
