chore: improve deal maintenance path

[lphuc2250gma]

Summary:

• Add narrow edge-case unit tests covering invariant (@deal.inv) behavior on classes with slots and on subclasses that override init, plus a couple of explicit type annotations on helper functions in deal/_imports.py that currently lack return-type hints.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.