fix(plan): unwrap format_output envelope in artifact JSON step assertions

The artifacts() method routes JSON through format_output() which wraps
the payload in a spec-required envelope {"data": ..., "status": ...}.
The two @tdd_issue_4253 step assertions were checking parsed["key"]
directly, but the actual fields live at parsed["data"]["key"].

Update step_artifacts_json_validation and step_artifacts_json_apply_summary
to extract parsed["data"] before asserting on validation_summary and
apply_summary respectively.

ISSUES CLOSED: #9084

features/steps/plan_diff_artifacts_steps.py

@@ -435,8 +435,9 @@ def step_artifacts_has_files(context: Context) -> None:
 @then("the artifacts JSON should contain validation summary")
 def step_artifacts_json_validation(context: Context) -> None:
     parsed = json.loads(context.artifacts_output)
-    assert "validation_summary" in parsed
-    assert parsed["validation_summary"]["total"] == 3
+    data = parsed["data"]
+    assert "validation_summary" in data
+    assert data["validation_summary"]["total"] == 3
 
 
 @then("the artifacts should show null changeset")
@@ -565,6 +566,7 @@ def step_plan_with_changeset_id_but_no_store_entry(context: Context) -> None:
 @then("the artifacts JSON should contain apply summary")
 def step_artifacts_json_apply_summary(context: Context) -> None:
     parsed = json.loads(context.artifacts_output)
-    assert "apply_summary" in parsed
-    assert parsed["apply_summary"]["files_changed"] == "7"
-    assert parsed["apply_summary"]["validations_run"] == "4"
+    data = parsed["data"]
+    assert "apply_summary" in data
+    assert data["apply_summary"]["files_changed"] == "7"
+    assert data["apply_summary"]["validations_run"] == "4"