Fix action execution server JSONResponse

[li-boxuan]

• This change is worth documenting at https://docs.all-hands.dev/
• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

End-user friendly description of the problem this fixes or functionality that this introduces.

---

Give a summary of what the PR does, explaining any non-trivial design decisions.

I was debugging action_execution_server on windows and found this bug. My debugging scenario:

1. Launching server: poetry run python -u -m openhands.runtime.action_execution_server 37234 --working-dir D:\workspace\ --plugins agent_skills jupyter --username boxuanli --user-id 1000
2. Calling API endpoint: curl -X POST -d '...payload...' http://localhost:37234/execute_action

Without converting to str, I sometimes see {"detail":"An unexpected error occurred. Please try again later."} from client side when there's an error message that contains bytes. On server side, the error is TypeError: Object of type bytes is not JSON serializable.

This PR fixes this bug by converting the error to string.

---

Link of any specific issues this addresses.

---

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:07ea4b3-nikolaik   --name openhands-app-07ea4b3   docker.all-hands.dev/all-hands-ai/openhands:07ea4b3

[xingyaoww]

Thanks!