Draft: dim end point is changed if dimension is constrained

### Problem description

The end point picked for a linear dimension is changed if the dimension is constrained to be horizontal or vertical.

### Workbench affected?

Draft

### Steps to reproduce

1. Restart in Safe Mode.
2. Start Draft_Dimension.
3. Pick 0,0 for the 1st point by snapping to the grid.
4. Pick 10,10 for the 2nd point.
5. Hold Shift and move the cursor above y=10.
6. The preview should show a horizontal dimension, click a point to confirm the position.
7. Result: End property of the dimension object is 10,0,0.

### Expected behavior

Picked points should be preserved, alignment should be achieved through the Direction property.

### Actual behavior

End point is changed.

### Development version About Info (in Safe Mode)

```shell
OS: Ubuntu 22.04.5 LTS (ubuntu:GNOME/ubuntu/wayland)
Architecture: x86_64
Version: 1.2.0dev.20260429 (Git shallow) AppImage
Build date: 2026/04/29 07:07:16
Build type: Release
Branch: grafted,grafted
Hash: a785d40ac4c37f559b0b82fbaf9bf7dc68a468e4
Python 3.11.14, Qt 6.8.3, Coin 4.0.8, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: Dutch/Netherlands (nl_NL)
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/86.1283/1
```

### Last known good version (optional)

```shell

```

Reproduce the issue in the Draft workbench using Draft_Dimension and the listed snapping and Shift-constrained steps. Compare the picked points with the dimension object's End and Direction properties; the work is complete when the picked end point is preserved while horizontal or vertical alignment is represented by Direction.

Written by the indexing model from the issue text.