Avoid mkdir(parents=True) in save_response

This can impliclity create a corrupted storage if it has been deleted in-flight

(cherry picked from commit 2760247)

Author: berland

src/ert/storage/local_ensemble.py

@@ -890,7 +890,7 @@ def save_response(
             )
 
         output_path = self._realization_dir(realization)
-        Path.mkdir(output_path, parents=True, exist_ok=True)
+        Path(output_path).mkdir(exist_ok=True)
 
         self._storage._to_parquet_transaction(
             output_path / f"{response_type}.parquet", data

tests/ert/unit_tests/storage/test_local_storage.py

@@ -1113,6 +1113,57 @@ def test_set_failure_will_not_recreate_ensemble_directory(storage):
     assert not dummy_ensemble._path.exists()
 
 
+def test_save_response_will_create_realization_directory(storage):
+    # Given a fresh ensemble storage with no realizations
+    dummy_ensemble = storage.create_experiment(
+        responses=[SummaryConfig(keys=["DUMMY"])]
+    ).create_ensemble(name="dummy", ensemble_size=1)
+    assert dummy_ensemble._path.exists(), "Assumptions for test has changed"
+    assert not (dummy_ensemble._path / "realization-0").exists()
+
+    # When a response is saved:
+    dummy_ensemble.save_response(
+        "summary",
+        pl.DataFrame(
+            {
+                "response_key": ["DUMMY"],
+                "time": pl.Series([datetime(2000, 1, 1)]).dt.cast_time_unit("ms"),
+                "values": pl.Series([0.0], dtype=pl.Float32),
+            }
+        ),
+        0,
+    )
+
+    # Then the realization directory was implicitly created
+    assert (dummy_ensemble._path / "realization-0").exists()
+    assert list(dummy_ensemble._path.glob("realization-0/*.parquet"))
+
+
+def test_save_response_will_not_recreate_ensemble_directory(storage):
+    dummy_ensemble = storage.create_experiment().create_ensemble(
+        name="dummy", ensemble_size=1
+    )
+    # Emulate a user deleting storage:
+    shutil.rmtree(dummy_ensemble._path)
+
+    # Function test will raise:
+    with pytest.raises(FileNotFoundError):
+        dummy_ensemble.save_response(
+            "summary",
+            pl.DataFrame(
+                {
+                    "response_key": ["DUMMY"],
+                    "time": pl.Series([datetime(2000, 1, 1)]).dt.cast_time_unit("ms"),
+                    "values": pl.Series([0.0], dtype=pl.Float32),
+                }
+            ),
+            0,
+        )
+
+    # and the ensemble path will not be created
+    assert not dummy_ensemble._path.exists()
+
+
 @dataclass
 class Ensemble:
     uuid: UUID