Write a function total_playlist_duration(durations: str) -> str that receives a single string containing a comma‑separated list of song lengths. Each song length is written as minutes:seconds (e.g. 3:15). The function must return the total length of the playlist, also formatted as minutes:seconds where seconds are always two digits.
Examples
total_playlist_duration("3:15, 4:05, 2:45") # -> "10:05"
total_playlist_duration("") # -> "0:00"