Sketcher: Error when creating redundant dimensions

### Problem description

This problem exists regardless of if the setting to auto-remove redundant constraints is enabled or not.

When creating a redundant dimension in sketcher, the programs throws an error. The problem can happen when a dimension is redundant with a constraint, but only if the dimension is created last.

### Workbench affected?

Sketcher

### Steps to reproduce

Case 1:

1. Create an arc (half circle) from sketch origin, with the two vertex constrained to the horizontal or vertical axis
2. Create a radius constraint
3. Create a constrain distance between the center and one of the vertex

Case 2:

1. Create two lines sharing a vertex
2. Create a perpendicular constraint between the lines
3. Create an angle dimension between the lines

Case 3:

1. Create two lines sharing a vertex
2. Create a 90° angle dimension between the lines
3. Create a perpendicular constraint between the lines

### Expected behavior

If the auto-remove redundant constraints option is enabled, then the last constraint is removed without any error.

If the auto-remove redundant constraints option is disabled, then the last constraint is kept, the sketch turns orange, and there is no error.

### Actual behavior

https://github.com/user-attachments/assets/b9a63aa7-bb0c-4d05-bc28-2ee6fbfb7b90

Case 1: same result regardless of auto-remove redundant option status

```
14:40:23 pyException: Traceback (most recent call last):
File "", line 1, in
: Datum 33,831 mm for the constraint with index 4 is invalid
14:40:23 Unnamed#Sketch: App.getDocument('Unnamed').getObject('Sketch').setDatum(4,App.Units.Quantity('33.831 mm'))
```

Case 2: same result regardless of auto-remove redundant option status

```
14:44:00 pyException: Traceback (most recent call last):
File "", line 1, in
: Datum 90,000° for the constraint with index 6 is invalid
14:44:00 Unnamed#Sketch: App.getDocument('Unnamed').getObject('Sketch').setDatum(6,App.Units.Quantity('90 deg'))
```

Case 3:

* if the option is enabled: perpendicular constraint is instantly removed as expected, no error
* if the option is enabled: the perpendicular is created, sketch turns orange as expected, no error

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

```shell
OS: Ubuntu 24.04.4 LTS (KDE/plasma/xcb)
Architecture: x86_64
Version: 26.3.0dev.20260825 (Git shallow) AppImage
Build date: 2026/08/25 17:50:34
Build type: Release
Branch: (HEAD detached at 964fce2ed)
Hash: 964fce2edc9a7e1905c2fd3fcd6e7be90afc8375
Python 3.11.14, Qt 6.8.3, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.0, OCC 7.8.1
Locale: French/France (fr_FR)
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
QPA/File dialog/Color dialog: xcb/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Drag at cursor
Logical DPI/Physical DPI/Pixel Ratio: 96/117.596/1
OpenGL version/vendor/renderer: 4.6 (Compatibility Profile) Mesa 25.2.8-0ubuntu0.24.04.2/Intel/Mesa Intel(R) Iris(R) Xe Graphics (TGL GT2)
Installed mods:
* Cables Workbench 0.3.6
* Curved Shapes 1.0.15
* Curves workbench 0.6.73
* DFM 0.1.16
* Fasteners Workbench 0.5.64
* SheetMetal Workbench 0.8.21
```

### Last known good version (optional)

```shell

```

Start in the Sketcher constraint-creation flow and follow the setDatum calls shown in the error output, reproducing the three cases with auto-remove redundant constraints both enabled and disabled. Done means redundant dimensions no longer raise a ValueError: the last constraint is removed when enabled, or retained with an orange sketch when disabled.

Written by the indexing model from the issue text.