Sketcher: Underconstrained sketch reported as BOTH overconstrained and fully constrained

### Problem description

I was designing a piece to fit a nema 17 shaft. So, a 5mm diameter arc connected to a flat spot.
Sketcher incorrectly reports my sketch as over-constrained, but still allows me to drag/resize part of it.
After dragging its geometry, sketcher reports the geometry as fully constrained, which it is not either.

### Workbench affected?

Sketcher

### Steps to reproduce

Create a new sketch (on whatever plane you want).

-Create a new line, across the vertical axis of the sketch.
-Apply Symmetry constraint, on its endpoints, against the vertical axis of the sketch.
-Create an arc, whose center is on the origin of the sketch and endpoints coincident with the line endpoints.

(Even at this point, if you drag a vertex, the sketch will sometimes turn red, and sketcher will complain it is over constrained, which it is clearly not !)

-Apply a radius constraint to the arc

[sketcher_bug_report.zip](https://github.com/user-attachments/files/31596780/sketcher_bug_report.zip)

### Expected behavior

The sketch should still be reported as underconstrained. The length of the line is still undefined.

### Actual behavior

Sketcher engine reports the sketch as "overconstrained", but I can still move parts of its geometry.
After I drag one of the two vertices, sketcher reports the sketch as "fully constrained" which is still incorrect.

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

```shell
I encountered this issue on both 1.1.13 and 2026.08.26 weekly.

OS: Debian GNU/Linux 13 (trixie) (KDE/plasma/wayland)
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 Dark/
QPA/File dialog/Color dialog: xcb/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: Blender/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/101.6/1
OpenGL version/vendor/renderer: 4.6.0 NVIDIA 610.57.04/NVIDIA Corporation/NVIDIA GeForce RTX 3060/PCIe/SSE2
```

### Last known good version (optional)

```shell

```

Start by reproducing the case in the Sketcher workbench using the line, symmetry, arc, coincidence, and radius constraints described. Trace how Sketcher determines the constraint state before and after dragging a vertex, then add a regression check showing that the line length remains undefined and the sketch stays underconstrained.

Written by the indexing model from the issue text.