- Added Windows-specific process handle duplication mechanism to safely manage process termination across multiple handles.
- Implemented Windows signal handling support with Signal enum variants (Terminate, Kill, Interrupt) and kill_process() function using Windows API.
- Added background job termination on cancellation with platform-specific implementations (TERM/KILL signals on Unix, TerminateProcess on Windows).
- Implemented cancellation token support in shell execution with proper cleanup of background jobs when timeout or abort occurs.
- Added test coverage for background job termination on timeout and abort scenarios.