fix(assembly): classify datum planes from all class hierarchies in Distance joints

The datum plane detection in getDistanceType() only checked for
App::Plane (origin planes). This missed two other class hierarchies:

  - PartDesign::Plane (inherits Part::Datum, NOT App::Plane)
  - Part::Plane primitive referenced bare (no Face element)

Both produce empty element types (sub-name ends with ".") but failed
the isDerivedFrom<App::Plane>() check, falling through to
DistanceType::Other and the Planar fallback. This caused incorrect
constraint geometry, leading to conflicting/unsatisfiable constraints
and solver failures.

Add shape-based isDatumPlane/Line/Point helpers that cover all three
hierarchies by inspecting the actual OCCT geometry rather than relying
on class identity alone. Extend getDistanceType() to use these helpers
for all datum-vs-datum and datum-vs-element combinations.

Adds TestDatumClassification.py with tests for PartDesign::Plane,
Part::Plane (bare ref), and cross-hierarchy datum combinations.

src/Mod/Assembly/AssemblyTests/TestDatumClassification.py

@@ -0,0 +1,266 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# /****************************************************************************
+#                                                                           *
+#    Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+#                                                                           *
+#    This file is part of FreeCAD.                                          *
+#                                                                           *
+#    FreeCAD is free software: you can redistribute it and/or modify it     *
+#    under the terms of the GNU Lesser General Public License as            *
+#    published by the Free Software Foundation, either version 2.1 of the   *
+#    License, or (at your option) any later version.                        *
+#                                                                           *
+#    FreeCAD is distributed in the hope that it will be useful, but         *
+#    WITHOUT ANY WARRANTY; without even the implied warranty of             *
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+#    Lesser General Public License for more details.                        *
+#                                                                           *
+#    You should have received a copy of the GNU Lesser General Public       *
+#    License along with FreeCAD. If not, see                                *
+#    <https://www.gnu.org/licenses/>.                                       *
+#                                                                           *
+# ***************************************************************************/
+
+"""
+Tests for datum plane classification in Distance joints.
+
+Verifies that getDistanceType correctly classifies joints involving datum
+planes from all three class hierarchies:
+  1. App::Plane          — origin planes (XY, XZ, YZ)
+  2. PartDesign::Plane   — datum planes inside a PartDesign body
+  3. Part::Plane         — Part workbench plane primitives (bare reference)
+"""
+
+import unittest
+
+import FreeCAD as App
+import JointObject
+
+
+class TestDatumClassification(unittest.TestCase):
+    """Tests that Distance joints with datum plane references are
+    classified as PlanePlane (not Other) regardless of the datum
+    object's class hierarchy."""
+
+    def setUp(self):
+        doc_name = self.__class__.__name__
+        if App.ActiveDocument:
+            if App.ActiveDocument.Name != doc_name:
+                App.newDocument(doc_name)
+        else:
+            App.newDocument(doc_name)
+        App.setActiveDocument(doc_name)
+        self.doc = App.ActiveDocument
+
+        self.assembly = self.doc.addObject("Assembly::AssemblyObject", "Assembly")
+        self.jointgroup = self.assembly.newObject("Assembly::JointGroup", "Joints")
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+
+    # ── Helpers ─────────────────────────────────────────────────────
+
+    def _make_box(self, x=0, y=0, z=0, size=10):
+        box = self.assembly.newObject("Part::Box", "Box")
+        box.Length = size
+        box.Width = size
+        box.Height = size
+        box.Placement = App.Placement(App.Vector(x, y, z), App.Rotation())
+        return box
+
+    def _make_joint(self, joint_type, ref1, ref2):
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, joint_type)
+        refs = [
+            [ref1[0], ref1[1]],
+            [ref2[0], ref2[1]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+        return joint
+
+    def _make_pd_body_with_datum_plane(self, name="Body"):
+        """Create a PartDesign::Body with a datum plane inside the assembly."""
+        body = self.assembly.newObject("PartDesign::Body", name)
+        datum = body.newObject("PartDesign::Plane", "DatumPlane")
+        self.doc.recompute()
+        return body, datum
+
+    def _make_part_plane(self, name="PartPlane"):
+        """Create a Part::Plane primitive inside the assembly."""
+        plane = self.assembly.newObject("Part::Plane", name)
+        plane.Length = 10
+        plane.Width = 10
+        self.doc.recompute()
+        return plane
+
+    # ── Origin plane tests (App::Plane — existing behaviour) ───────
+
+    def test_origin_plane_face_classified_as_plane_plane(self):
+        """Distance joint: box Face → origin datum plane → PlanePlane."""
+        origin = self.assembly.Origin
+        xy = origin.getXY()
+        box = self._make_box(0, 0, 50)
+
+        joint = self._make_joint(
+            5,  # Distance
+            [box, ["Face1", "Vertex1"]],
+            [origin, [xy.Name + ".", xy.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertEqual(
+            result,
+            0,
+            "Distance joint with origin plane should solve (not produce Other)",
+        )
+
+    # ── PartDesign::Plane tests ────────────────────────────────────
+
+    def test_pd_datum_plane_face_classified_as_plane_plane(self):
+        """Distance joint: box Face → PartDesign::Plane → PlanePlane."""
+        body, datum = self._make_pd_body_with_datum_plane()
+        box = self._make_box(0, 0, 50)
+
+        # Ground the body so the solver has a fixed reference.
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, body)
+
+        # Reference the datum plane with a bare sub-name (ends with ".").
+        joint = self._make_joint(
+            5,  # Distance
+            [box, ["Face1", "Vertex1"]],
+            [body, [datum.Name + ".", datum.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint with PartDesign::Plane should not fail to solve "
+            "(DistanceType should be PlanePlane, not Other)",
+        )
+
+    def test_pd_datum_plane_vertex_classified_as_point_plane(self):
+        """Distance joint: box Vertex → PartDesign::Plane → PointPlane."""
+        body, datum = self._make_pd_body_with_datum_plane()
+        box = self._make_box(0, 0, 50)
+
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, body)
+
+        joint = self._make_joint(
+            5,  # Distance
+            [box, ["Vertex1", "Vertex1"]],
+            [body, [datum.Name + ".", datum.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint vertex → PartDesign::Plane should not fail "
+            "(DistanceType should be PointPlane, not Other)",
+        )
+
+    def test_two_pd_datum_planes_classified_as_plane_plane(self):
+        """Distance joint: PartDesign::Plane → PartDesign::Plane → PlanePlane."""
+        body1, datum1 = self._make_pd_body_with_datum_plane("Body1")
+        body2, datum2 = self._make_pd_body_with_datum_plane("Body2")
+
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, body1)
+
+        joint = self._make_joint(
+            5,  # Distance
+            [body1, [datum1.Name + ".", datum1.Name + "."]],
+            [body2, [datum2.Name + ".", datum2.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint PartDesign::Plane → PartDesign::Plane should not fail "
+            "(DistanceType should be PlanePlane, not Other)",
+        )
+
+    # ── Part::Plane tests (primitive, bare reference) ──────────────
+
+    def test_part_plane_bare_ref_face_classified_as_plane_plane(self):
+        """Distance joint: box Face → Part::Plane (bare ref) → PlanePlane."""
+        plane = self._make_part_plane()
+        box = self._make_box(0, 0, 50)
+
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, plane)
+
+        # Bare reference to Part::Plane (sub-name ends with ".").
+        joint = self._make_joint(
+            5,  # Distance
+            [box, ["Face1", "Vertex1"]],
+            [plane, [".", "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint with Part::Plane (bare ref) should not fail "
+            "(DistanceType should be PlanePlane, not Other)",
+        )
+
+    def test_part_plane_with_face1_classified_as_plane_plane(self):
+        """Distance joint: box Face → Part::Plane Face1 → PlanePlane.
+
+        When Part::Plane is referenced with an explicit Face1 element,
+        it should enter the normal Face+Face classification path."""
+        plane = self._make_part_plane()
+        box = self._make_box(0, 0, 50)
+
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, plane)
+
+        joint = self._make_joint(
+            5,  # Distance
+            [box, ["Face1", "Vertex1"]],
+            [plane, ["Face1", "Vertex1"]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint with Part::Plane Face1 should solve normally",
+        )
+
+    # ── Cross-hierarchy tests ──────────────────────────────────────
+
+    def test_origin_plane_and_pd_datum_classified_as_plane_plane(self):
+        """Distance joint: origin App::Plane → PartDesign::Plane → PlanePlane."""
+        origin = self.assembly.Origin
+        xy = origin.getXY()
+        body, datum = self._make_pd_body_with_datum_plane()
+
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, body)
+
+        joint = self._make_joint(
+            5,  # Distance
+            [origin, [xy.Name + ".", xy.Name + "."]],
+            [body, [datum.Name + ".", datum.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertNotEqual(
+            result,
+            -1,
+            "Distance joint origin plane → PartDesign::Plane should not fail "
+            "(DistanceType should be PlanePlane, not Other)",
+        )