test(download): cover playlist_position in payload serialization test

test/models_and_utils_test.dart

@@ -311,6 +311,7 @@ void main() {
         postProcessingEnabled: true,
         tidalHighFormat: 'opus_256',
         trackNumber: 7,
+        playlistPosition: 3,
         discNumber: 2,
         totalTracks: 12,
         totalDiscs: 2,
@@ -358,6 +359,7 @@ void main() {
         'post_processing_enabled': true,
         'tidal_high_format': 'opus_256',
         'track_number': 7,
+        'playlist_position': 3,
         'disc_number': 2,
         'total_tracks': 12,
         'total_discs': 2,