turns out, im impatient as hell. so i brute forced it!

any item with a custom texture has these two attributes : (defindex:152) hi and (defindex:227) lo

Its the decal id split into 32 bits (32 high bits,32 lower bits)

you can reconstruct it like this:

def reverse_seed(lo, hi):
    # Combine the high and low parts to reconstruct the original seed
    seed = (lo << 32) + hi
    return seed

Pretty easy! then you have to pass it through https://api.steampowered.com/ISteamRemoteStorage/GetUGCFileDetails/v1/?key={STEAM_API_KEY}&ugcid={seed}&appid=440 To get the full url!

there's probably an easier way, but i don't care. it works.