openai/gpt-5.2-chat → A (3:1)

Side A adds a defined `GITHUB_API_BASE_URL` environment variable with a safe default and `.rstrip("/")`, directly fixing a runtime NameError in the GitHub OAuth callback path. Side B mainly addresses clippy warnings and minor test/idiomatic cleanups (e.g., pattern matching simplifications, `is_some_and`, formatting tweaks) without changing functionality, so its impact is largely cosmetic and maintenance-oriented.