Fix mypy errors in openhands/llm directory

[neubig]

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

• Adding proper type annotations to functions
• Fixing ModelInfo type issues
• Fixing ModelResponse type issues
• Fixing async iterator type issues
• Adding proper return type annotations
• Fixing decorator type issues

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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:5266187-nikolaik   --name openhands-app-5266187   docker.all-hands.dev/all-hands-ai/openhands:5266187