Fix type annotations and mypy compliance

Author: OzGav

music_assistant/controllers/media/albums.py

@@ -458,19 +458,23 @@ async def _set_album_artist(
         self, db_id: int, artist: Artist | ItemMapping, overwrite: bool = False
     ) -> ItemMapping:
         """Store Album Artist info."""
-        db_artist: Artist | ItemMapping
+        db_artist: Artist | ItemMapping | None = None
         if artist.provider == "library":
             db_artist = artist
         elif existing := await self.mass.music.artists.get_library_item_by_prov_id(
             artist.item_id, artist.provider
         ):
             db_artist = existing
-        elif isinstance(artist, Artist):
-            db_artist = await self.mass.music.artists.add_item_to_library(
-                artist, overwrite_existing=overwrite
-            )
-        else:
-            db_artist = artist
+
+        if not db_artist or overwrite:
+            # Type narrowing: if artist is an ItemMapping, convert it or handle it
+            if isinstance(artist, ItemMapping):
+                # ItemMapping can't be added directly, use the existing or skip
+                db_artist = artist
+            else:
+                db_artist = await self.mass.music.artists.add_item_to_library(
+                    artist, overwrite_existing=overwrite
+                )
         # write (or update) record in album_artists table
         assert self.mass.music.database is not None
         await self.mass.music.database.insert_or_replace(