Fix AStars to return empty path for disabled from point

[kleonc]

Fixes #113975.

#86983 changed only AStar2D::get_id_path/AStar3D::get_id_path to return an empty path if from point is disabled.

This PR makes the same for:

• AStar2D::get_point_path
• AStar3D::get_point_path
• AStarGrid2D::get_id_path (API-wise for AStarGrid2D points can be "solid" instead of "disabled")
• AStarGrid2D::get_point_path

For all AStars, I've moved the condition checking to the internal _solve method used by both get_id_path/get_point_path.

Also previously (even after #86983) e.g. AStar2D::get_id_path(from, from) for a disabled from would still return a single-point path instead of an empty path, as from == to was being checked before !from.enabled. Fixed/swapped these checks order as well.

Added relevant test case for AStar3D (AStar2D, AStarGrid2D have no tests at all).

[ellie1232]

Tested in engine in 2D, Confirmed the issue exists on the current stable branch and that it behaves as expected in the PR.
Read over the commit and didn't see any obvious issues but I'm not overly familiar with the source code so I may have missed something.