Add test for vf_ground_sky_2d view factor bounds

[Chirag3841]

• [x ] Closes bifacial.utils.vf_ground_sky_2d interprets max_rows asymmetrically #1867
• [ x] I am familiar with the contributing guidelines
• [ x] Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• [ x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [ x] Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Description

Fixes an off-by-one / fencepost issue in vf_ground_sky_2d where one fewer sky
wedge was considered on the right side compared to the left side in symmetric
geometries.

The fix includes horizon bounds (cos(0)=1 and cos(pi)=-1) so the wedge
calculation is consistent on both sides.

Tests

Added a regression test test_vf_ground_sky_2d_returns_valid_range to ensure
the returned view factor stays within the physically valid range [0, 1].

[Chirag3841]

@AdamRJensen I implemented a fix for the fencepost/off-by-one issue in vf_ground_sky_2d
and added related regression test too.