Add strict mypy checking

[neubig]

Make mypy checking more strict.

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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:78c89a3-nikolaik   --name openhands-app-78c89a3   docker.all-hands.dev/all-hands-ai/openhands:78c89a3

[raymyers]

Generally happy to see this change and if I can be of help chipping away at some of the fixes let me know.

I haven't tried them but there are various reports mypy can run on the thoroughness of type coverage, might be worth a look if not already using.

[mamoodi]

@neubig a gentle ping in case this fell off your radar.

[neubig]

I'm going to abandon this because we have more important priorities.