Fix resolver linting issues

[neubig]

This PR fixes several linting issues in the resolver code:

• Add base_branch field to GithubIssue class to fix attr-defined error
• Fix api_key type handling in LLMConfig instantiation by properly converting to str

All linting checks now pass for the resolver directory.

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:d48f0ed-nikolaik   --name openhands-app-d48f0ed   docker.all-hands.dev/all-hands-ai/openhands:d48f0ed

[malhotra5]

LGTM, thanks!