Run runtime.close in background thread

[tofarr]

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End-user friendly description of the problem this fixes or functionality that this introduces.
Fix for issue where closing a conversation blocks the main runloop

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Give a summary of what the PR does, explaining any non-trivial design decisions.
Users had reported intermittent stalls, and I had observed this but not been able to pin it down. Eventually with some careful use of time.sleep(), I was able to narrow the stall down to the session close, and particularly the runtime.close. This will no longer block the main runloop.

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Link of any specific issues this addresses.

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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:11f5198-nikolaik   --name openhands-app-11f5198   docker.all-hands.dev/all-hands-ai/openhands:11f5198