add fp1 sample

csharp-fp1/Domain/InsufficientBalanceException.cs

@@ -0,0 +1,17 @@
+namespace CsharpFp1.Domain;
+
+/// Custom domain exception thrown when a withdrawal would cause the balance to go below zero
+public sealed class InsufficientBalanceException : InvalidOperationException
+{
+    public Money CurrentBalance { get; }
+    public Money RequestedAmount { get; }
+
+    public InsufficientBalanceException(Money currentBalance, Money requestedAmount)
+        : base(
+            $"Insufficient balance. Current: {currentBalance.Amount:0.00}, Requested: {requestedAmount.Amount:0.00}"
+        )
+    {
+        CurrentBalance = currentBalance;
+        RequestedAmount = requestedAmount;
+    }
+}