Function select_utxos 

pub fn select_utxos(
    utxos: &[ChronikUtxo],
    target_sats: i64,
) -> Result<CoinSelection, String>

Select UTXOs to cover target_sats using largest-first strategy.

Only considers UTXOs that have no SLP/ALP token metadata (plain XEC UTXOs). Returns Err if insufficient funds.