Fix mypy errors in core directory

[neubig]

This PR fixes all mypy errors in the core directory by:

1. Removing all type: ignore comments and fixing the underlying issues
2. Properly handling dynamic attributes in LogRecord
3. Fixing type issues with ImageContent.model_dump() return value
4. Fixing role type issues in Message class

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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:54e50d0-nikolaik   --name openhands-app-54e50d0   docker.all-hands.dev/all-hands-ai/openhands:54e50d0