fix(tmdb): harden aiohttp session lifecycle

- Add async context manager to NFOService wrapping TMDBClient + ImageDownloader
- Add TMDBClient.__del__ warning when session leaks
- Log exc_info on session recreation for traceback visibility
- Document async-with usage in docs/DEVELOPMENT.md and docs/TESTING.md
- Add unit tests covering leak detection, context-manager cleanup, and connector-closed warning

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/core/services/nfo_service.py

@@ -53,6 +53,18 @@ class NFOService:
         self.image_size = image_size
         self.auto_create = auto_create
     
+    async def __aenter__(self) -> "NFOService":
+        """Enter async context manager."""
+        await self.tmdb_client.__aenter__()
+        await self.image_downloader.__aenter__()
+        return self
+    
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+        """Exit async context manager and cleanup resources."""
+        await self.tmdb_client.close()
+        await self.image_downloader.close()
+        return False
+    
     def has_nfo(self, serie_folder: str) -> bool:
         """Check if tvshow.nfo exists for a series.