test: fix NFO workflow and background loader tests

- Add missing TMDB async mock methods (_ensure_session, close)
  to all TMDB mocks in test_nfo_workflow.py
- Refactor test_anime_add_nfo_isolation.py to mock get_nfo_factory()
  instead of asserting on series_app.nfo_service directly
- Patch get_nfo_factory in test_background_loader_service.py
  to align with factory-based NFOService creation

Fixes test failures caused by NFOService refactoring that introduced
explicit TMDB session lifecycle and NFO factory pattern.

tests/unit/test_background_loader_service.py

@@ -520,19 +520,25 @@ class TestLoadNfoAndImages:
         mock_db = AsyncMock()
         mock_series = MagicMock()
         mock_series.has_nfo = False
-        
+
         task = SeriesLoadingTask(
             key="test",
             folder="test_folder",
             name="Test Series",
             year=2020
         )
-        
-        with patch("src.server.database.service.AnimeSeriesService") as mock_service_class:
+
+        mock_nfo_service = AsyncMock()
+        mock_nfo_service.create_tvshow_nfo = AsyncMock(return_value="/anime/test_folder/tvshow.nfo")
+        mock_factory = MagicMock()
+        mock_factory.create = MagicMock(return_value=mock_nfo_service)
+
+        with patch("src.server.database.service.AnimeSeriesService") as mock_service_class, \
+             patch("src.server.services.background_loader_service.get_nfo_factory", return_value=mock_factory):
             mock_service_class.get_by_key = AsyncMock(return_value=mock_series)
-            
+
             result = await background_loader_service._load_nfo_and_images(task, mock_db)
-        
+
         assert result is True
         assert task.progress["nfo"] is True
         assert task.progress["logo"] is True