Fix mypy errors in storage directory

[neubig]

This PR fixes mypy errors in the storage directory by:

• Adding proper return type annotations to functions in conversation_validator.py
• Removing unnecessary # type: ignore comments in google_cloud.py
• Removing unnecessary # type: ignore comment in s3.py

All changes are strictly related to fixing type errors and improving type safety. There are no functional changes or unrelated modifications.

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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:bf14d5e-nikolaik   --name openhands-app-bf14d5e   docker.all-hands.dev/all-hands-ai/openhands:bf14d5e