fix: Ability to create fee with negative units or amount (#2966)

We still have some invoices stuck in generating status, when units or
amount are negative.

The goal of this PR is to permit the creation of the fee by converting
the value to 0 when it's negative for `units` and `amount` fields.

But we keep the calculation details on `amount_details`.

Example:
<img width="961" alt="Screenshot 2024-12-17 at 10 12 46"
src="https://github.com/user-attachments/assets/01270a74-bdcb-4e1a-a811-19c52455d049"
/>

app/services/fees/charge_service.rb

@@ -107,6 +107,12 @@ def init_fee(amount_result, properties:, charge_filter:)
         return
       end
 
+      # Prevent trying to create a fee with negative units or amount.
+      if amount_result.units.negative? || amount_result.amount.negative?
+        amount_result.amount = amount_result.unit_amount = BigDecimal(0)
+        amount_result.full_units_number = amount_result.units = amount_result.total_aggregated_units = BigDecimal(0)
+      end
+
       # NOTE: amount_result should be a BigDecimal, we need to round it
       # to the currency decimals and transform it into currency cents
       rounded_amount = amount_result.amount.round(currency.exponent)