fix: keep InstalledPackagesFeatureGroup's output shape on the error path

mloda_plugins/feature_group/experimental/environment/installed_packages_feature_group.py

@@ -91,7 +91,8 @@ class InstalledPackagesFeatureGroup(FeatureGroup):
     - Uses `subprocess.run()` to execute `pip freeze`
     - Captures stdout as text
     - Returns packages as a single string in a list (DataFrame-compatible)
-    - On error, returns error message dictionary
+    - On error, returns the error message in that same column, so the output shape
+      does not change between the success and failure paths
 
     ## Security Considerations
 
@@ -108,7 +109,7 @@ def calculate_feature(cls, data: Any, features: FeatureSet) -> Any:
             return {cls.get_class_name(): [packages]}
         except subprocess.CalledProcessError as e:
             error_message = f"Command '{e.cmd}' failed with return code {e.returncode}. Error output: {e.stderr}"
-            return {"error": error_message}
+            return {cls.get_class_name(): [error_message]}
 
     @classmethod
     def compute_framework_rule(cls) -> set[type[ComputeFramework]]:

tests/test_plugins/feature_group/experimental/environment/test_installed_packages_feature_group.py

@@ -1,3 +1,6 @@
+import subprocess
+from unittest.mock import patch
+
 from mloda.user import Feature
 from mloda.provider import FeatureSet
 from mloda_plugins.feature_group.experimental.environment.installed_packages_feature_group import (
@@ -19,3 +22,24 @@ def test_installed_packages_feature_group_mlodaAPI() -> None:
     result = mloda.run_all(features, compute_frameworks={PandasDataFrame})
     assert len(result) == 1
     assert InstalledPackagesFeatureGroup.get_class_name() in result[0]
+
+
+def test_installed_packages_feature_group_error_path_keeps_the_output_shape() -> None:
+    """A pip freeze failure must land in the documented column, not a separate "error" key.
+
+    The success path returns {ClassName: [...]}, and the class docstring's Output
+    Format section promises that single column. A different key on failure gives a
+    caller a KeyError instead of a readable message, exactly when it is debugging.
+    """
+    feature_set = FeatureSet()
+    failure = subprocess.CalledProcessError(returncode=1, cmd=["pip", "freeze"], stderr="boom")
+
+    with patch("subprocess.run", side_effect=failure):
+        result = InstalledPackagesFeatureGroup.calculate_feature(None, feature_set)
+
+    column = InstalledPackagesFeatureGroup.get_class_name()
+    assert set(result) == {column}, "the failure path must not introduce another key"
+    assert isinstance(result[column], list)
+    assert len(result[column]) == 1
+    assert "boom" in result[column][0]
+    assert "return code 1" in result[column][0]