The problem was basically that, after efd3c4b3c9, `test_cagep` started
to be used without being calculated.
`test_cagep` represents the closest 3D point on an edge.
The solution was to edit the `knife_snap_edge_constrained` function to
calculate the `test_cagep` instead of the `closest_ss` (which is the
projected point).
Calculating `test_cagep`(3D) instead of `closest_ss`(2D) is
advantageous as we avoid matrix transformations and achieve more
precision.
We also deduplicate the code a bit since `closest_ss` can be obtained
by projecting `test_cagep`.
This commit also adds comments to the code, and renamed some variables
and functions for more clarity.
Pull Request: https://projects.blender.org/blender/blender/pulls/124701
7249b78b6b