Refactor: Github Service

[malhotra5]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

The motivation for this PR is "separation of concerns" with respect to the GithubService class

We want to the GithubService to

• Execute all Github API requests
• Return typed objects (dicts, lists, etc) or raise appropriate exceptions to ensure reliability
• Never be responsible for constructing JSONResponses for its callers. This is for reusability purposes. The caller of the GithubService class should be responsible for handling the data and constructing the JSONResponses if need be

This PR accomplishes these constraints by -

1. Removing the GithubTokenMiddleware; token fetching for the user occurs inside GithubService
2. Returning only typed objects from GithubService
3. Having the FastAPI endpoints be responsible for constructing JSONResponse objects, rather than delegating that responsibility to GithubService

This achieves "separation of concerns", and better reusability because we can use GithubService in both HTTP and non-HTTP related settings

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Link of any specific issues this addresses

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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:3e6c96b-nikolaik   --name openhands-app-3e6c96b   docker.all-hands.dev/all-hands-ai/openhands:3e6c96b

[tofarr]

🍰 - Just one question (Not a blocker)