fix(infra): update transaction amount handling to use CanonicalAmount and ensure negative values in transaction tests

src/FiscalOS.Infra/Transactions/Plaid/PlaidAddedTransactionHandler.cs

@@ -89,7 +89,7 @@ internal sealed class PlaidAddedTransactionHandler : IPlaidAddedTransactionHandl
           account.Id,
           addedTransaction.Merchant,
           addedTransaction.Description,
-          addedTransaction.Amount,
+          addedTransaction.CanonicalAmount,
           addedTransaction.PostedDate,
           transactionMetadata
         );