Refactor refund logic to use UtilHelper for formatting XRPL amounts

[du1ana]

When refunding tenants, the decimal multiplication of leaseAmount * remainingMoments can produce values exceeding XRPL's precision limits, causing Decimal precision out of range errors.

Steps to Reproduce

1. Set a minimal lease amount (e.g., 0.000001 EVRs) on an Evernode host
2. Create an instance using ev-devkit
3. Delete the instance using evernode delete

Note: This error occurs intermittently, depending on the lease amount and remaining moment count at the time of deletion.

Error

Refunding tenant ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░...
Error: Decimal precision out of range
    at Amount.assertIouIsValid (...)

Fix

Introduced UtilHelper.formatXrplAmount() to ensure XRPL-compliant precision (15 significant digits) when calculating refund amounts.

Before:

(uriInfo.leaseAmount * remainingMoments).toString()

After:

UtilHelper.formatXrplAmount(uriInfo.leaseAmount, remainingMoments)