fix: ensure scaling for different reserveAsset token decimals (#551)

### Description

This PR adds scaling to the reserve balance read in
mintUBIFromReserveBalance in Case the token has less than 18 decimals.

### Other changes



### Tested

- added test verifying expected behavior for token with 6 decimals

### Related issues


### Backwards compatibility


### Documentation

contracts/goodDollar/GoodDollarExpansionController.sol

@@ -5,6 +5,7 @@ import { IGoodDollarExpansionController } from "contracts/interfaces/IGoodDollar
 import { IGoodDollarExchangeProvider } from "contracts/interfaces/IGoodDollarExchangeProvider.sol";
 import { IBancorExchangeProvider } from "contracts/interfaces/IBancorExchangeProvider.sol";
 import { IERC20 } from "openzeppelin-contracts-next/contracts/token/ERC20/IERC20.sol";
+import { IERC20Metadata } from "openzeppelin-contracts-next/contracts/token/ERC20/extensions/IERC20Metadata.sol";
 import { IGoodDollar } from "contracts/goodDollar/interfaces/IGoodProtocol.sol";
 import { IDistributionHelper } from "contracts/goodDollar/interfaces/IGoodProtocol.sol";
 
@@ -152,7 +153,9 @@ contract GoodDollarExpansionController is IGoodDollarExpansionController, Ownabl
     IBancorExchangeProvider.PoolExchange memory exchange = IBancorExchangeProvider(address(goodDollarExchangeProvider))
       .getPoolExchange(exchangeId);
 
-    uint256 contractReserveBalance = IERC20(exchange.reserveAsset).balanceOf(reserve);
+    uint256 contractReserveBalance = IERC20(exchange.reserveAsset).balanceOf(reserve) *
+      (10 ** (18 - IERC20Metadata(exchange.reserveAsset).decimals()));
+
     uint256 additionalReserveBalance = contractReserveBalance - exchange.reserveBalance;
     if (additionalReserveBalance > 0) {
       amountMinted = goodDollarExchangeProvider.mintFromInterest(exchangeId, additionalReserveBalance);

test/unit/goodDollar/GoodDollarExpansionController.t.sol

@@ -5,6 +5,7 @@ pragma solidity 0.8.18;
 
 import { Test } from "forge-std/Test.sol";
 import { ERC20Mock } from "openzeppelin-contracts-next/contracts/mocks/ERC20Mock.sol";
+import { ERC20DecimalsMock } from "openzeppelin-contracts-next/contracts/mocks/ERC20DecimalsMock.sol";
 import { GoodDollarExpansionController } from "contracts/goodDollar/GoodDollarExpansionController.sol";
 
 import { IGoodDollarExpansionController } from "contracts/interfaces/IGoodDollarExpansionController.sol";
@@ -326,6 +327,37 @@ contract GoodDollarExpansionControllerTest_mintUBIFromReserveBalance is GoodDoll
     assertEq(amountMinted, 0);
   }
 
+  function test_mintUBIFromReserveBalance_whenReserveAssetDecimalsIsLessThan18_shouldScaleCorrectly() public {
+    ERC20DecimalsMock reserveToken6DecimalsMock = new ERC20DecimalsMock("Reserve Token", "RES", 6);
+    IBancorExchangeProvider.PoolExchange memory pool2 = IBancorExchangeProvider.PoolExchange({
+      reserveAsset: address(reserveToken6DecimalsMock),
+      tokenAddress: address(token),
+      tokenSupply: 7 * 1e9 * 1e18,
+      reserveBalance: 200_000 * 1e18, // internally scaled to 18 decimals
+      reserveRatio: 0.2 * 1e8, // 20%
+      exitContribution: 0.1 * 1e8 // 10%
+    });
+
+    uint256 reserveInterest = 1000e6;
+    deal(address(reserveToken6DecimalsMock), reserveAddress, 200_000 * 1e6 + reserveInterest);
+
+    vm.mockCall(
+      address(exchangeProvider),
+      abi.encodeWithSelector(IBancorExchangeProvider(exchangeProvider).getPoolExchange.selector, exchangeId),
+      abi.encode(pool2)
+    );
+
+    vm.expectCall(
+      address(exchangeProvider),
+      abi.encodeWithSelector(
+        IGoodDollarExchangeProvider(exchangeProvider).mintFromInterest.selector,
+        exchangeId,
+        reserveInterest * 1e12
+      )
+    );
+    expansionController.mintUBIFromReserveBalance(exchangeId);
+  }
+
   function test_mintUBIFromReserveBalance_whenAdditionalReserveBalanceIsLargerThan0_shouldMintAndEmit() public {
     uint256 amountToMint = 1000e18;
     uint256 additionalReserveBalance = 1000e18;