fix(lidarr): wrap MBID-lookup decode errors with ErrInvalidPayload + fill test gaps

- Decode errors in LookupArtist/AlbumByMBID now wrap ErrInvalidPayload so
  callers can errors.Is the same way they do against the M5a methods.
- Tighten TestLookupAlbumByMBID_BadJSON to assert on ErrInvalidPayload.
- Add empty-mbid rejection tests for both methods.
- Add TestLookupArtistByMBID_EmptyArrayReturnsErrNotFound (only the album
  variant had the test before — symmetric coverage now).
- Normalize the network-error test's api key to key123 for consistency
  with the rest of delete_test.go.

internal/lidarr/lookup_mbid.go

@@ -22,7 +22,7 @@ func (c *Client) LookupArtistByMBID(ctx context.Context, mbid string) (LidarrArt
 
 	var rows []LidarrArtist
 	if err := json.NewDecoder(resp.Body).Decode(&rows); err != nil {
-		return LidarrArtist{}, fmt.Errorf("lidarr: decode artist: %w", err)
+		return LidarrArtist{}, fmt.Errorf("%w: decode artist: %v", ErrInvalidPayload, err)
 	}
 	if len(rows) == 0 {
 		return LidarrArtist{}, ErrNotFound
@@ -45,7 +45,7 @@ func (c *Client) LookupAlbumByMBID(ctx context.Context, mbid string) (LidarrAlbu
 
 	var rows []LidarrAlbum
 	if err := json.NewDecoder(resp.Body).Decode(&rows); err != nil {
-		return LidarrAlbum{}, fmt.Errorf("lidarr: decode album: %w", err)
+		return LidarrAlbum{}, fmt.Errorf("%w: decode album: %v", ErrInvalidPayload, err)
 	}
 	if len(rows) == 0 {
 		return LidarrAlbum{}, ErrNotFound