Fix type hints: compute_face_areas order param and return type

[rajeeja]

Noted by @Sevans711 during review of #1577 — these issues already exist on main, not introduced by #1577.

1. compute_face_areas and related signatures use order: int | None = 4 (or Optional[int]) but None is not a valid value — should be order: int = 4.
2. Return type annotation missing on compute_face_areas.

Narrow cleanup PR welcome.

[Sevans711]

Similarly, the other kwargs include None as an option but None is not a valid value. For example, quadrature_rule: str | None = "triangular" should be quadrature_rule: str = "triangular", and latitude_adjusted_area: bool | None = False should be latitude_adjusted_area: bool = False.

Actually, a quick search for the string "| None = " throughout the codebase reveals many similar issues with allowing None when it should not be allowed because the code does not handle it. I will open a separate issue for that, so that this one can remain narrowly focused on face_areas related type hinting.