[Github Integration] Authflow will return 500 if passing in an invalid oauth code

It seems error in code exchange isn't handled correctly, therefore the user info endpoint returned 401 which cause 500 in our api.

The issue mentions the GitHub OAuth code-exchange flow and user-info endpoint; start by tracing that path through the 401 response for an invalid code. Done means the invalid OAuth code is handled without the API returning 500, with the resulting behavior verified through the relevant existing test path.

Written by the indexing model from the issue text.