test: cover unknown error_type ValueError path

tests/unit/test_split_selection.py

@@ -1,3 +1,5 @@
+import pytest
+
 from app.harness.split_selection import cell_of, evolution_target_of
 
 
@@ -8,5 +10,10 @@ def test_evolution_target_mapping():
     assert evolution_target_of("mixed") == "system"
 
 
+def test_evolution_target_unknown_raises():
+    with pytest.raises(ValueError):
+        evolution_target_of("unknown_type")
+
+
 def test_cell_is_task_type_x_error_type():
     assert cell_of("Counting Problem", "search_failure") == ("Counting Problem", "search_failure")