add test project

csharp-fp2.Tests/WithdrawMoneyHandlerTests.cs

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+using CsharpFp2.Applications;
+using CsharpFp2.Contracts;
+using CsharpFp2.Domain;
+using CsharpFp2.Infrastructure;
+
+namespace CsharpFp2.Tests;
+
+/// Tests covering the application-layer use case surface:
+/// what the WithdrawMoney feature does from the caller's perspective.
+public class WithdrawMoneyHandlerTests
+{
+    private static (
+        WithdrawMoneyHandler handler,
+        InMemoryAccountRepository repository
+    ) BuildHandler()
+    {
+        var repository = new InMemoryAccountRepository();
+        var handler = new WithdrawMoneyHandler(repository);
+        return (handler, repository);
+    }
+
+    [Fact]
+    public void Withdrawing_from_an_account_reduces_its_balance_by_the_withdrawn_amount()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(200m)));
+
+        handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 75m });
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(125m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_the_entire_balance_leaves_the_account_at_zero()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(100m)));
+
+        handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 100m });
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(0m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_from_a_non_existent_account_throws()
+    {
+        var (handler, _) = BuildHandler();
+        var command = new WithdrawMoneyCommand { AccountId = Guid.NewGuid(), Amount = 50m };
+
+        Assert.Throws<InvalidOperationException>(() => handler.Handle(command));
+    }
+
+    [Fact]
+    public void Withdrawing_more_than_the_available_balance_throws()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(50m)));
+
+        Assert.Throws<InsufficientBalanceException>(() =>
+            handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 100m })
+        );
+    }
+
+    [Fact]
+    public void After_a_failed_withdrawal_the_balance_is_unchanged()
+    {
+        var (handler, repository) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repository.Save(new Account(new AccountId(accountId), new Money(50m)));
+
+        try
+        {
+            handler.Handle(new WithdrawMoneyCommand { AccountId = accountId, Amount = 999m });
+        }
+        catch (InsufficientBalanceException) { }
+
+        var account = repository.GetById(new AccountId(accountId))!;
+        Assert.Equal(50m, account.Balance.Amount);
+    }
+}