test(wallet): cover TransactionFactory's standardness-policy rejection

builder.Verify's failure branch (the safety net right before a signed
tx would be returned to the caller) had no test: an absurd fee rate
now proves the "Invalid transaction: ..." path fires instead of
silently returning a policy-violating transaction.

tests/PalladiumWallet.Tests/Wallet/TransactionFactoryTests.cs

@@ -104,6 +104,21 @@ public class TransactionFactoryTests
         Assert.Contains("unconfirmed", ex.Message);
     }
 
+    [Fact]
+    public void Una_fee_oltre_la_policy_standard_viene_rifiutata_prima_del_broadcast()
+    {
+        // An absurd fee rate produces a fee far above NBitcoin's standard policy
+        // cap: the builder.Verify safety net must refuse the transaction instead
+        // of letting a fat-finger fee reach the network.
+        var account = Account();
+        var (utxos, txs) = Fund(account, 100_000_000); // 1 PLM
+
+        var ex = Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(1), amountSats: 1_000_000,
+            feeRateSatPerVByte: 500_000, changeIndex: 0, tipHeight: 100));
+        Assert.Contains("Invalid transaction", ex.Message);
+    }
+
     [Fact]
     public void Gli_utxo_congelati_sono_esclusi_dalla_spesa()
     {