Fix OVPhysX iface fixture to mirror PhysX/Newton setup

The fixture was calling data._process_cfg(obj.cfg), but _process_cfg
moved from RigidObjectData onto RigidObject in commit 0887d2d
("Mirror PhysX/Newton patterns in OVPhysX RigidObject"). PhysX/Newton
fixtures don't populate default_root_pose / default_root_vel at all,
so drop the call.

The fixture was also missing the asset-side buffers (_ALL_INDICES,
_ALL_BODY_INDICES, _ALL_TRUE_*_MASK, _wrench_buf, _cpu_* pinned
staging) that RigidObject._initialize_impl normally allocates via
_create_buffers. The PhysX / Newton fixtures install these manually;
on the OVPhysX side it is simpler to call obj._create_buffers()
directly and then overwrite the real WrenchComposers with mocks.

Brings the failing 372 cross-backend iface tests to passing
(372 ovphysx-backend cases now green; 1476 rigid-object iface tests
total pass with 12 documented xfails).

Author: AntoineRichard

source/isaaclab/test/assets/test_rigid_object_iface.py

@@ -257,11 +257,16 @@ def create_ovphysx_rigid_object(
     data = OvPhysxRigidObjectData(mock_bindings.bindings, device)
     data.num_instances = num_instances
     data.num_bodies = 1
-    data._process_cfg(obj.cfg)
     data._is_primed = True
     object.__setattr__(obj, "_data", data)
 
-    # Wrench composers
+    # Build the buffers RigidObject normally allocates in _initialize_impl
+    # (_ALL_INDICES, _ALL_*_MASK, pinned CPU staging buffers, wrench buf).
+    # _create_buffers also instantiates real WrenchComposers; those get
+    # replaced with mocks just below.
+    obj._create_buffers()
+
+    # Replace the real wrench composers with mocks for iface coverage.
     mock_inst_wrench = MockWrenchComposer(obj)
     mock_perm_wrench = MockWrenchComposer(obj)
     object.__setattr__(obj, "_instantaneous_wrench_composer", mock_inst_wrench)