Fix mypy errors in storage directory

[neubig]

This PR fixes all mypy errors in the openhands/storage directory by:

• Adding return type annotations to functions that were missing them
• Fixing return type issues in google_cloud.py and s3.py by ensuring string returns

Changes are limited to the storage directory only.

Part of #6397

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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:5d8d946-nikolaik   --name openhands-app-5d8d946   docker.all-hands.dev/all-hands-ai/openhands:5d8d946