Add process termination to stop button functionality

[neubig]

This PR adds process termination functionality to the stop button. When the stop button is clicked:

1. The agent state is changed to STOPPED
2. The runtime terminates all processes in its environment:
   • LocalRuntime: Uses ps and kill to manage local processes
   • DockerRuntime: Uses pkill inside the container
   • ModalRuntime: Uses sandbox termination
   • RemoteRuntime: Uses API calls

Each runtime handles process termination in a way appropriate to its environment, with graceful termination (SIGTERM) followed by force kill (SIGKILL) if needed.

Tested with:

• Processes that handle SIGTERM gracefully
• Processes that ignore SIGTERM
• Nested processes (parent/child)
• Multiple concurrent processes

Fixes #6848

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:411cfa9-nikolaik   --name openhands-app-411cfa9   docker.all-hands.dev/all-hands-ai/openhands:411cfa9