test: verify state file not updated when HTML generation fails

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_incremental_snapshot.py

@@ -157,3 +157,19 @@ def test_fetch_failure_does_not_update_state(tmp_path):
 
     assert d.load_last_run_date() == "2025-01-01"
     assert d.load_snapshot_cache() == [{"id": "existing"}]
+
+
+def test_html_generation_failure_does_not_update_state_file(tmp_path):
+    d = _downloader(tmp_path)
+    d.save_last_run_date("2025-01-01")
+    d.save_snapshot_cache([{"id": "existing"}])
+    new_snapshots = [{"id": "new", "startTime": "2025-02-01T00:00:00Z"}]
+    with patch.object(d, "authenticate", new_callable=AsyncMock):
+        with patch.object(d, "fetch_all_snapshots", new_callable=AsyncMock,
+                          return_value=new_snapshots):
+            with patch.object(d, "generate_html_file", new_callable=AsyncMock,
+                              side_effect=OSError("disk full")):
+                with pytest.raises(OSError):
+                    asyncio.run(d.download_snapshots(date_from="2024-01-01"))
+    # Cache was updated with new data, but state file was NOT advanced
+    assert d.load_last_run_date() == "2025-01-01"