openai/gpt-chat-latest → A (9:1)

Side A performs a substantial refactor that consolidates duplicated test infrastructure into shared utilities (`test.common` and `test.oauth`), adds reusable helpers such as `run-cargo-build-release!`, `slug-server-env`, `complete-registration!`, and parameterized mock Google behavior, and updates multiple integration test suites to use them. Side B mainly adds a planning document and introduces a thin `RouteContext` wrapper re-export without migrating call sites, so it provides architectural intent but little immediate functional value.