chore: improve vyper maintenance path

[lphuc2250gma]

Summary:

• Add or tighten focused edge-case tests or type assertions in tests/hevm.py, tests/utils.py related to Python typing, tests, CLI ergonomics, observability; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

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