[BUG] VDB Analysis Closest Point doesnt seem to move outer boundary points accurately

### Environment
**Operating System:** (e.g. Windows 11)
**Version / Commit SHA:** (e.g Latest VDB that comes with Houdini 19.5)
**Other:** (e.g compiler, C++ standard etc.)

### Describe the bug
VDB Analysis Closest Point doesnt seem to move outer boundary points accurately.

### To Reproduce
Steps to reproduce the behavior:
I submitted this to SideFX and got this answer where he commented on your source code:

"I stashed the original positions and then lerped between original & computed positions. This let me see it is the points in the outer boundary that aren't moving all the way in.

I then looked at the VDB code,
https://github.com/AcademySoftwareFoundation/openvdb/blob/master/openvdb/openvdb/tools/GridOperators.h
is where Cpt() is created, this then runs CPT() on all the voxels, the latter is in
https://github.com/AcademySoftwareFoundation/openvdb/blob/master/openvdb/openvdb/math/Operators.h

Therein you'll see code like this:
// compute gradient in physical space where it is a unit normal
// since the grid holds a distance level set.
Vec3d vectorFromSurface(d*Gradient::result(map, grid, ijk));
if (is_linear::value) {
Vec3d result = ijk.asVec3d() - map.applyInverseMap(vectorFromSurface);
return Vec3Type(result);
} else {
Vec3d location = map.applyMap(ijk.asVec3d());
Vec3d result = map.applyInverseMap(location - vectorFromSurface);
return Vec3Type(result);
}

Noticeably absent from this code is an normalization of the gradient. This means that at the outer ring of active voxels where the sdf clamps to the border value; the gradient will drop from what it is in the bandwidth. This means the voxel values at the boundary have less than unit gradients, so undershoot the surface.
"

### Expected behavior
The question of whether CPT should normalize the gradient to get a better answer for the outermost row of voxels.

### Additional context
(Add any other context about the problem here.)

Start with openvdb/tools/GridOperators.h and openvdb/math/Operators.h, following Cpt() into CPT() and the gradient calculation described in the report. Build or locate a reproducible case for outer-boundary voxels, then compare the computed closest points with the expected surface positions. Done means the boundary behavior is explained and covered by an agreed fix or regression test.

Written by the indexing model from the issue text.