Update Hermitian.decomposition test to use qml.matrix (#6095)

After the merge of #6062 , the legacy op math tests in the plugin test
matrix fail (see
[here](https://github.com/PennyLaneAI/pennylane/actions/runs/10362479768/job/28685492331)),
since the test uses the `Operator.matrix` method to verify the
decomposition, but the decomposed operator is a legacy `Hamiltonian`
with new op math disabled, which doesn't have a matrix. This PR just
updates the test to use `qml.matrix` instead of `Operator.matrix`.

tests/ops/qubit/test_observables.py

@@ -369,8 +369,8 @@ def test_hermitian_decomposition(self, observable):
         )
         A_decomp = qml.Hermitian(observable, wires=list(range(num_wires))).decomposition()
 
-        assert np.allclose(A_decomp_static[0].matrix(), observable, rtol=0)
-        assert np.allclose(A_decomp[0].matrix(), observable, rtol=0)
+        assert np.allclose(qml.matrix(A_decomp_static[0]), observable, rtol=0)
+        assert np.allclose(qml.matrix(A_decomp[0]), observable, rtol=0)
 
     @pytest.mark.parametrize("observable, eigvals, eigvecs", EIGVALS_TEST_DATA)
     def test_hermitian_diagonalizing_gates(self, observable, eigvals, eigvecs, tol, mocker):