chore: improve rust-bindgen maintenance path

[vip892766gma]

Summary:

• Add focused unit tests for IntKind/VarType helpers (e.g., signedness, known-size mapping, and edge cases like u128/i128 and bool) to lock in current behavior and guard against regressions; small, mechanical additions in #[cfg(test)] modules without touching public APIs.
• Keep the change narrow so it is straightforward to review.

Notes:

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