Fix Github service bugs

[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

This PR fixes a group of bugs. NOTE: this doesn't affect any existing functionality. However, these changes are blocking for implementing token refresh logic

The bugs fixed

1. Headers should be dynamically generated (due to token refresh)
2. App mode comparison is always false (replace "SAAS" with AppMode.SAAS)
3. Bad response for get_installation_ids function means a dict object isn't return. This PR handles this edge case appropriately
4. We should use fetch_response to call APIs that require refresh token logic (this PR fixed one such case where get_user was used in place of fetch_response('get_user')

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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:5af19c7-nikolaik   --name openhands-app-5af19c7   docker.all-hands.dev/all-hands-ai/openhands:5af19c7

[tofarr]

Could we expose a load_user method that returns the user and raises an exception? (It feels weird to have the internals of the github integration exposed here.)

[tofarr]

🍰 - I see you have these suggestions already in subsequent PRs.