Fix face_areas type hints and add compute_face_areas return type

[SAY-5]

Closes #1590

Overview

• calculate_total_face_area, compute_face_areas, and _compute_face_areas annotated quadrature_rule, order, and latitude_adjusted_area with | None, but None is never a valid value and the code does not handle it. Dropped the | None so the annotations match the actual accepted types (str, int, bool).
• Added the missing return type annotation tuple[np.ndarray, np.ndarray] to compute_face_areas (and _compute_face_areas), matching what they return and the existing docstrings.

Kept narrowly scoped to the face_areas signatures per @Sevans711's note; the broader codebase-wide sweep is tracked separately.

Expected Usage

PR Checklist

General

• An issue is linked created and linked
• [N/A] Add appropriate labels
• Filled out Overview and Expected Usage (if applicable) sections

Testing

• [N/A] Adequate tests are created if there is new functionality
• [N/A] Tests cover all possible logical paths in your function
• [N/A] Tests are not too basic (such as simply calling a function and nothing else)

Documentation

• [N/A] Docstrings have been added to all new functions
• Docstrings have updated with any function changes
• [N/A] Internal functions have a preceding underscore (_) and have been added to docs/internal_api/index.rst
• [N/A] User functions have been added to docs/user_api/index.rst

Examples

• [N/A] Any new notebook examples added to docs/examples/ folder
• [N/A] Clear the output of all cells before committing
• [N/A] New notebook files added to docs/examples.rst toctree
• [N/A] New notebook files added to new entry in docs/gallery.yml with appropriate thumbnail photo in docs/_static/thumbnails/

[Sevans711]

Thank you @SAY-5 for taking the time to look into this and for making this clean pull request!

@rajeeja what would be your preferences on how to proceed? I think the fix looks good here when compared to main, but I know that the changes on #1577 would change the return type for compute_face_areas to something more complicated (something like: np.ndarray | UxDataArray | tuple[np.ndarray | UxDataArray, np.ndarray]). One option is to merge this fix to main as-is, and update the return type appropriately on #1577. Another option is to delay here and fix here after the other PR merges to main.

[rajeeja]

Thanks for the fix @SAY-5, and nice contribution for a first PR here. This resolves #1590 cleanly.

@Sevans711 — let's go ahead and merge this one now; I'll handle the small conflict with #1577 when I merge main in.

@SAY-5 out of curiosity, what got you into using uxarray? Always good to hear how people find the project.

[rajeeja]

LGTM — clean, correctly scoped fix. Approving.