feat: add exactOptional support in type struct

[ccharly]

Add support of exactOptional with superstruct.type.

We plan to use @metamask/superstruct in @metamask/keyring-api and we already use the combo type + exactOptional (our implementation) there.

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Note

Low Risk
Small, localized change to type() entry iteration aligned with existing object() semantics; covered by new tests with no auth or data-path impact.

Overview
Extends type() so exactOptional() fields behave like TypeScript’s optional properties: the key may be missing, but an explicit undefined value is still validated and rejected.

The type() entries generator now skips properties wrapped in exactOptional when the key is not on the object (hasOwnProperty), matching the existing object() behavior.

Adds typing coverage for type + exactOptional, plus validation fixtures for absent keys, present values, and age: undefined failing validation.

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