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

tests/FiscalOS.Infra.Tests/Integration/PlaidAddedTransactionHandlerTests.cs

@@ -30,6 +30,9 @@ public class PlaidAddedTransactionHandlerTests : IntegrationTest
 
     result.Should().Be(1);
     account.Transactions.Should().HaveCount(2);
+    account.Transactions
+      .Single(t => t.Metadata is PlaidTransactionMetadata m && m.PlaidId == transaction.TransactionId)
+      .Amount.Should().Be(-transaction.Amount);
   }
 
   [Fact]

tests/FiscalOS.Infra.Tests/Integration/PlaidModifiedTransactionHandlerTests.cs

@@ -31,5 +31,6 @@ public class PlaidModifiedTransactionHandlerTests : IntegrationTest
 
     result.Should().Be(1);
     existingTransaction.MerchantName.Should().Be(transaction.MerchantName);
+    existingTransaction.Amount.Should().Be(-transaction.Amount);
   }
 }