fix(PlaylistService): 곡 순서 보장되도록 수정

Author: gzhan0226

main-server/src/main/java/com/example/demo/domain/playlist/service/PlaylistSaveService.java

@@ -91,7 +91,7 @@ public PlaylistWithSongsResponse editPlaylistWithSongs(
 
         // 3) 응답
         List<SongResponseDto> songDtos = songRepository
-                .findByPlaylistId(playlistId)
+                .findSongsByPlaylistId(playlistId)
                 .stream()
                 .map(SongMapper::toDto)
                 .toList();