tests: a failed hold step refuses a real direct stepper call

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_moving_bodies.py

@@ -321,6 +321,29 @@ def test_failed_advance_fails_simulation(monkeypatch, failure):
         sim.run(1, stream=STREAM)
 
 
+def test_failed_hold_step_blocks_the_direct_stepper(monkeypatch):
+    # No target is pending, so only the failure latch can refuse direct steps.
+    from CelerisLab.lbm.stepper import LBMStepper
+    sim, writes, motion = advancing_simulation(monkeypatch, fail="upload")
+    motion._on_failure = sim._fail_moving
+    launches = []
+    kernels = SimpleNamespace(get_function=lambda name: lambda *a, **k: launches.append(name))
+    cfg = SimpleNamespace(streaming="double_buffer", threads_per_block=128, is_d2q9=True,
+                          nx=64, ny=32, nz=1)
+    field = SimpleNamespace(ddf_gpu="ddf", temp_gpu="temp", flag_gpu="flag", n_curved=0)
+    stepper = LBMStepper(field, kernels, cfg)
+    stepper.moving = sim.stepper.moving
+    sim.stepper = stepper
+    assert motion.pending is None and stepper.blocked is None
+    with pytest.raises(RuntimeError, match="upload failed"):
+        sim.run(2, stream=STREAM)  # hold step
+    assert sim._failed is not None and stepper.blocked == sim._failed
+    with pytest.raises(RuntimeError, match="moving circle step failed"):
+        stepper.step(1, action_gpu=None, obs_gpu=None)
+    assert launches == [] and stepper.step_count == 0
+    assert (field.ddf_gpu, field.temp_gpu) == ("ddf", "temp")
+
+
 def test_moving_tables_layout_and_attachment(monkeypatch):
     from CelerisLab.body.motion import MovingTables
     a = MovingCircle((50., 40.), 10., Translation((-1., 1., -3., 3.), .1))