Assembly: Make pre-solve and reverse move all the downstream parts (#24193)

src/Mod/Assembly/App/AssemblyObject.pyi

@@ -148,5 +148,28 @@ class AssemblyObject(Part):
         Args:
         fileName: The name of the file where the ASMT will be exported."""
         ...
+
+    @constmethod
+    def getDownstreamParts(
+        self, start_part: "App.DocumentObject", joint_to_ignore: "App.DocumentObject", /
+    ) -> list["App.DocumentObject"]:
+        """
+        Finds all parts connected to a start_part that are not connected to ground
+        when a specific joint is ignored.
+
+        getDownstreamParts(start_part, joint_to_ignore) -> list
+
+        This is used to find the entire rigid group of unconstrained components that
+        should be moved together during a pre-solve operation or a drag.
+
+        Args:
+            start_part: The App.DocumentObject to begin the search from.
+            joint_to_ignore: The App.DocumentObject (a joint) to temporarily
+                             suppress during the connectivity check.
+
+        Returns:
+            A list of App.DocumentObject instances representing the downstream parts.
+        """
+        ...
     Joints: Final[list]
     """A list of all joints this assembly has."""

src/Mod/Assembly/App/AssemblyObjectPyImp.cpp

@@ -198,3 +198,36 @@ Py::List AssemblyObjectPy::getJoints() const
 
     return ret;
 }
+
+PyObject* AssemblyObjectPy::getDownstreamParts(PyObject* args) const
+{
+    PyObject* pyPart;
+    PyObject* pyJoint;
+
+    // Parse the two arguments: a part object and a joint object
+    if (!PyArg_ParseTuple(args,
+                          "O!O!",
+                          &(App::DocumentObjectPy::Type),
+                          &pyPart,
+                          &(App::DocumentObjectPy::Type),
+                          &pyJoint)) {
+        return nullptr;
+    }
+
+    auto* part = static_cast<App::DocumentObjectPy*>(pyPart)->getDocumentObjectPtr();
+    auto* joint = static_cast<App::DocumentObjectPy*>(pyJoint)->getDocumentObjectPtr();
+
+    // Call the C++ method
+    std::vector<Assembly::ObjRef> downstreamParts =
+        this->getAssemblyObjectPtr()->getDownstreamParts(part, joint);
+
+    // Convert the result into a Python list of DocumentObjects
+    Py::List ret;
+    for (const auto& objRef : downstreamParts) {
+        if (objRef.obj) {
+            ret.append(Py::Object(objRef.obj->getPyObject(), true));
+        }
+    }
+
+    return Py::new_reference_to(ret);
+}