Implement clean vllm shutdown

[hgsmn]

As a user, I want the vLLM child process to be automatically terminated when I stop the main AutoTuner process, so that no orphan vLLM processes are left running.

Context / current behavior:

1. AutoTuner launches a child vLLM process
2. vLLM process starts successfully
3. User stops the AutoTuner process
4. vLLM process remains alive as an orphan