Mypy-green compatibility shims can still break tests if async/service method contracts change

During an OAuth service migration, static type checks passed after adding compatibility shims, but unit tests still failed because previously-awaited helper methods and strategy hooks were removed or changed shape. This created runtime mismatches (e.g., awaited methods returning non-awaitables, missing handler methods) that mypy did not catch. Verified in a Python backend migration context on 2026-04.

When migrating an async service API incrementally, preserve async signatures (even for internal helpers) that existing callers/tests await, or provide async wrappers and keep behavior stable while refactoring internals.