test: Add test coverage for steady-state warmup step increment

Add an additional forward() call in test_warmup_step_counter to cover
line 207 in _stch.py (the else branch that increments step during
steady-state operation after warmup completes).

Author: rkhosrowshahi

tests/unit/aggregation/test_stch.py

@@ -219,9 +219,12 @@ def test_warmup_step_counter():
     weighting(gramian)
     assert weighting.step == 3
 
-    weighting(gramian)
+    weighting(gramian)  # First step after warmup: nadir_vector gets computed
     assert weighting.step == 4
 
+    weighting(gramian)  # Steady-state: nadir_vector is already set
+    assert weighting.step == 5
+
 
 def test_warmup_with_varying_gramians():
     """Test warmup with different gramians to verify accumulation."""