Fix #126156: Do not allow to assign embedded IDs to IDProperties.

This is just not supported, and typically not doable from Blender UI
(since embedded IDs won't be listed in ID template list).

Add asserts to IDProp code itself, and checks & error reports in
relevant BPY and RNA setters.

source/blender/python/generic/idprop_py_api.cc

@@ -980,6 +980,12 @@ static IDProperty *idp_from_DatablockPointer(IDProperty *prop_exist,
   IDProperty *prop = nullptr;
   ID *value = nullptr;
   pyrna_id_FromPyObject(ob, &value);
+
+  if (value && (value->flag & ID_FLAG_EMBEDDED_DATA) != 0) {
+    PyErr_SetString(PyExc_ValueError, "Cannot assign an embedded ID pointer to an id-property");
+    return nullptr;
+  }
+
   if (prop_exist) {
     if (prop_exist->type == IDP_ID) {
       IDP_AssignID(prop_exist, value, 0);