1a2e833974
- 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.