Reduce conversion memory footprint

tests/test_storage.py

@@ -0,0 +1,34 @@
+from pathlib import Path
+
+from app.services.storage import persist_result
+
+
+def test_persist_result_writes_csv_and_collects_preview(tmp_path: Path):
+    metadata, export_result = persist_result(
+        output_dir=tmp_path,
+        records=[
+            {
+                "v015xxxxdate": "2024-05-01",
+                "time": "10:00:00",
+                "policy": "Prod Policy",
+                "severity_level": "high",
+            },
+            {
+                "v015xxxxdate": "2024-05-02",
+                "time": "11:00:00",
+                "policy": "Other Policy",
+                "severity_level": "low",
+            },
+        ],
+        union_keys=["v015xxxxdate", "time", "policy", "severity_level"],
+        mode="full",
+        output_format="csv",
+        preview_record_limit=1,
+    )
+
+    written = Path(metadata.file_path).read_text(encoding="utf-8")
+
+    assert metadata.download_name == "waf-report.csv"
+    assert "v015xxxxdate,time,policy,severity_level" in written
+    assert "2024-05-01,10:00:00,Prod Policy,high" in written
+    assert export_result.preview(1).count("\n") == 1