fix: add Drop to TaskManagerHandle to kill child processes on unexpec…

[abdalla1912mohamed]

…ted exit

• Add Drop impl for TaskManagerHandle that sends on the broadcast shutdown channel, triggering TaskManager::run() to call shutdown_all_tasks() which kills every process group
• Fix TaskManager::run() to also call shutdown_all_tasks() when the mpsc channel closes (all handles dropped without explicit shutdown), preventing orphaned child processes
• Extract duplicate process kill logic into terminate_process_group() helper used by both cancel_task and shutdown_all_tasks
• Give TaskManagerHandle a clone of the broadcast shutdown_tx (previously #[allow(dead_code)] and unused on TaskManager)
• Add tests: handle drop triggers shutdown, handle drop kills child processes