openai/gpt-chat-latest → A (5:1)

Side A fixes a runtime bug by defining `GITHUB_API_BASE_URL` with a default (`https://api.github.com`) and normalizing it via `rstrip("/")`, directly preventing a NameError during the GitHub OAuth callback when the environment variable is absent. Side B is largely a refactoring of test code, extracting shared helpers (`test.common`/`test.oauth`) and reducing duplication, which improves maintainability but does not materially change production behavior.