Use matrix mul to compose rotations in test_pink_ik

calculate_rotation_error used element-wise '*' between two 3x3 rotation
matrices where matrix multiplication '@' was intended. The Hadamard
product of two rotation matrices is not generally a rotation matrix --
its columns are not orthonormal and its determinant is not 1 -- so the
quat_from_matrix consumer downstream produced either a non-unit garbage
quaternion (pre-isaac-sim#5609) or NaN
(after the unit-norm guard added in that PR).

The bug was masked for ~9 months because the Hadamard product of two
near-identity matrices is also near-identity, so the resulting quat was
close enough to unit for quat_from_matrix to round-trip cleanly. Once
IK didn't converge to literal identity -- different seed, different
robot, more aggressive setpoint -- the assertion 'Left hand IK rotation
error (nan) exceeds tolerance' started firing on develop.

Local reproduction on isaaclab_physx with newton rc2 (current develop):
- Unfixed: Isaac-PickPlace-GR1T2-Abs-v0 horizontal_movement fails with
  NaN rotation error on both hands.
- Fixed: same parameterization passes with rotation errors at 1e-4 to
  1e-7, well within the 0.02 rad tolerance. 11 of 11 GR1T2 cases pass
  deterministically across re-runs.

Bug introduced by
isaac-sim#3149.

Author: hujc7

source/isaaclab/changelog.d/jichuanh-pink-ik-left-hand-nan.rst

@@ -0,0 +1,14 @@
+Fixed
+^^^^^
+
+* Fixed ``calculate_rotation_error`` in ``source/isaaclab/test/controllers/test_pink_ik.py``
+  using element-wise multiplication (``*``) instead of matrix multiplication (``@``) to
+  compose two rotation matrices, producing a non-rotation matrix and propagating ``NaN``
+  through ``quat_from_matrix`` (after the unit-norm guard added by
+  `isaac-sim/IsaacLab#5609 <https://github.com/isaac-sim/IsaacLab/pull/5609>`_). The latent
+  bug was introduced in `isaac-sim/IsaacLab#3149
+  <https://github.com/isaac-sim/IsaacLab/pull/3149>`_ and masked for ~9 months because the
+  Hadamard and matrix products of two near-identity rotation matrices are close enough that
+  ``quat_from_matrix`` could still return a near-unit quaternion. Once IK no longer
+  converged to literal identity (e.g., G1 envs or any seed perturbation), the assertion
+  ``Left hand IK rotation error (nan) exceeds tolerance`` started firing.

source/isaaclab/test/controllers/test_pink_ik.py

@@ -306,7 +306,7 @@ def calculate_rotation_error(current_rot, target_rot):
             target_rot_tensor = target_rot_tensor.unsqueeze(0).expand(current_rot.shape[0], -1)
 
     return axis_angle_from_quat(
-        quat_from_matrix(matrix_from_quat(target_rot_tensor) * matrix_from_quat(quat_inv(current_rot)))
+        quat_from_matrix(matrix_from_quat(target_rot_tensor) @ matrix_from_quat(quat_inv(current_rot)))
     )