Add a sanity test for load_app_config and get_agent_config_arg

[li-boxuan]

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

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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

Follow-up of #6662 (comment)

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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:04eba26-nikolaik   --name openhands-app-04eba26   docker.all-hands.dev/all-hands-ai/openhands:04eba26

[enyst]

Perfectly on point, thank you!