Fix miscalculated preview 2PT charge in review pt2 (#1584)

https://eaflood.atlassian.net/browse/WATER-4849

In [Fix miscalculated preview 2PT charge in review](#1583) we attempted to resolve an issue where the two-part tariff preview charge feature was incorrectly telling the [Charging Module API](https://github.com/DEFRA/sroc-charging-module-api) to include the £141 water company charge in its calculation.

We deduced the issue as logic based on whether a field was populated rather than incorporating whether that value was 'true' or 'false'.

What we didn't clock until testing was that when the value is populated, it is a boolean value. However, our fetch service is casting it to text, which means our fix still equates `isSupplyPublicWater = false` as 'true' because the value it is testing is `'false'` and not `false`.

This updates the fetch service to cast the value properly, which means the logic we amended in part 1 will now behave as expected.

app/services/bill-runs/review/fetch-review-charge-reference.service.js

@@ -69,7 +69,7 @@ async function _fetch(reviewChargeReferenceId) {
           'volume',
           'loss',
           ref('chargeReferences.additionalCharges:supportedSource.name').castText().as('supportedSourceName'),
-          ref('chargeReferences.additionalCharges:isSupplyPublicWater').castText().as('waterCompanyCharge')
+          ref('chargeReferences.additionalCharges:isSupplyPublicWater').castBool().as('waterCompanyCharge')
         ])
         .withGraphFetched('chargeCategory')
         .modifyGraph('chargeCategory', (chargeCategoryBuilder) => {

test/services/bill-runs/review/fetch-review-charge-reference.test.js → test/services/bill-runs/review/fetch-review-charge-reference.service.test.js

@@ -105,7 +105,7 @@ describe('Bill Runs Review - Fetch Review Charge Reference service', () => {
           volume: 6.819,
           loss: 'low',
           supportedSourceName: 'Foo source',
-          waterCompanyCharge: 'true',
+          waterCompanyCharge: true,
           chargeCategory: {
             id: chargeCategory.id,
             reference: chargeCategory.reference,