Function selectInputsForAmount

Select compressed inputs for a target amount.

Sorts by amount descending (largest first), accumulates until the target is met, then pads to MAX_INPUT_ACCOUNTS if possible within a single batch.

• If the amount is covered by N <= 8 inputs, returns min(8, total) inputs.
• If more than 8 inputs are needed, returns exactly as many as required (no padding beyond the amount-needed count).
• Returns [] when neededAmount <= 0 or accounts is empty.