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
         );

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

@@ -7,6 +7,7 @@ public static class PlaidExtensions
   {
     public string Merchant => transaction.MerchantName ?? "Unknown merchant";
     public string Description => transaction.OriginalDescription ?? "";
+    public decimal? CanonicalAmount => transaction.Amount is decimal amount ? -amount : null;
     public DateTimeOffset PostedDate => transaction.Datetime ?? (
         transaction.Date.HasValue
           ? new DateTimeOffset(

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

@@ -84,7 +84,7 @@ internal sealed class PlaidModifiedTransactionHandler : IPlaidModifiedTransactio
           existing.AccountId,
           plaidModifiedTransaction.Merchant,
           plaidModifiedTransaction.Description,
-          plaidModifiedTransaction.Amount,
+          plaidModifiedTransaction.CanonicalAmount,
           plaidModifiedTransaction.PostedDate,
           plaidMetadata
         );