fix(report): eliminate temp file leak in MD fallback; simplify invalid-format test

backend/app/api/report.py

@@ -5,7 +5,6 @@ Provides simulation report generation, retrieval, and chat endpoints
 
 import io
 import os
-import tempfile
 import traceback
 import threading
 import markdown as md_lib
@@ -480,12 +479,12 @@ def download_report(report_id: str):
                     as_attachment=True,
                     download_name=f"{report_id}.md"
                 )
-            with tempfile.NamedTemporaryFile(mode='w', suffix='.md',
-                                             delete=False, encoding='utf-8') as f:
-                f.write(markdown_content)
-                temp_path = f.name
-            return send_file(temp_path, as_attachment=True,
-                             download_name=f"{report_id}.md")
+            return send_file(
+                io.BytesIO(markdown_content.encode('utf-8')),
+                mimetype='text/markdown; charset=utf-8',
+                as_attachment=True,
+                download_name=f"{report_id}.md"
+            )
 
         # fmt == 'pdf'
         pdf_bytes = _generate_pdf_bytes(markdown_content)

backend/tests/test_report_download.py

@@ -80,13 +80,7 @@ class TestDownloadPDF:
 
         assert resp.status_code == 404
 
-    def test_download_pdf_invalid_format(self, client, tmp_path):
+    def test_download_pdf_invalid_format(self, client):
         """Returns 400 for unknown format parameter."""
-        mock_report = _make_mock_report()
-        md_path = _make_md_file(tmp_path, mock_report.report_id, mock_report.markdown_content)
-
-        with patch('app.api.report.ReportManager.get_report', return_value=mock_report), \
-             patch('app.api.report.ReportManager._get_report_markdown_path', return_value=md_path):
-            resp = client.get(f'/api/report/{mock_report.report_id}/download?format=docx')
-
+        resp = client.get('/api/report/any_id/download?format=docx')
         assert resp.status_code == 400