Test: add comprehensive unit test project for services

Add MoneyMap.Tests project with xUnit tests for all new services:

- AccountServiceTests: Account retrieval, stats, validation, deletion
- CardServiceTests: Card retrieval, stats, validation, deletion
- MerchantServiceTests: Merchant CRUD operations
- ReferenceDataServiceTests: Reference data retrieval
- TransactionServiceTests: Duplicate detection, retrieval, deletion
- TransactionStatisticsServiceTests: Statistics calculations
- ReceiptMatchingServiceTests: Receipt-to-transaction matching logic
- DbContextHelper: In-memory database context factory for test isolation

Uses xUnit, Moq, and EF Core InMemory database. Solution file updated to include test project.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap.Tests/Services/MerchantServiceTests.cs

@@ -0,0 +1,189 @@
+using MoneyMap.Models;
+using MoneyMap.Services;
+using MoneyMap.Tests.TestHelpers;
+using Xunit;
+
+namespace MoneyMap.Tests.Services;
+
+public class MerchantServiceTests
+{
+    [Fact]
+    public async Task GetOrCreateAsync_CreatesNewMerchant_WhenDoesNotExist()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        // Act
+        var result = await service.GetOrCreateAsync("Walmart");
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("Walmart", result.Name);
+        Assert.Single(context.Merchants);
+    }
+
+    [Fact]
+    public async Task GetOrCreateAsync_ReturnsExistingMerchant_WhenExists()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var existingMerchant = new Merchant { Id = 1, Name = "Walmart" };
+        context.Merchants.Add(existingMerchant);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.GetOrCreateAsync("Walmart");
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal(1, result.Id);
+        Assert.Single(context.Merchants);
+    }
+
+    [Fact]
+    public async Task GetAllMerchantsWithStatsAsync_ReturnsMerchantsWithStats()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var account = new Account
+        {
+            Id = 1,
+            Institution = "Test Bank",
+            AccountType = AccountType.Checking,
+            Last4 = "1234",
+            Owner = "Test Owner"
+        };
+        context.Accounts.Add(account);
+
+        var merchant1 = new Merchant { Id = 1, Name = "Walmart" };
+        var merchant2 = new Merchant { Id = 2, Name = "Target" };
+        context.Merchants.AddRange(merchant1, merchant2);
+
+        var transaction = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -50.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            MerchantId = 1
+        };
+        context.Transactions.Add(transaction);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.GetAllMerchantsWithStatsAsync();
+
+        // Assert
+        Assert.Equal(2, result.Count);
+        var walmart = result.First(m => m.Name == "Walmart");
+        Assert.Equal(1, walmart.TransactionCount);
+        var target = result.First(m => m.Name == "Target");
+        Assert.Equal(0, target.TransactionCount);
+    }
+
+    [Fact]
+    public async Task UpdateMerchantAsync_SuccessfullyUpdatesName()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var merchant = new Merchant { Id = 1, Name = "Walmart" };
+        context.Merchants.Add(merchant);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.UpdateMerchantAsync(1, "Walmart Supercenter");
+
+        // Assert
+        Assert.True(result.Success);
+        var updated = await context.Merchants.FindAsync(1);
+        Assert.Equal("Walmart Supercenter", updated!.Name);
+    }
+
+    [Fact]
+    public async Task UpdateMerchantAsync_FailsWhenDuplicateName()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var merchant1 = new Merchant { Id = 1, Name = "Walmart" };
+        var merchant2 = new Merchant { Id = 2, Name = "Target" };
+        context.Merchants.AddRange(merchant1, merchant2);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.UpdateMerchantAsync(1, "Target");
+
+        // Assert
+        Assert.False(result.Success);
+        Assert.Contains("already exists", result.Message);
+    }
+
+    [Fact]
+    public async Task DeleteMerchantAsync_SuccessfullyDeletes()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var merchant = new Merchant { Id = 1, Name = "Walmart" };
+        context.Merchants.Add(merchant);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.DeleteMerchantAsync(1);
+
+        // Assert
+        Assert.True(result.Success);
+        Assert.Empty(context.Merchants);
+    }
+
+    [Fact]
+    public async Task DeleteMerchantAsync_ReportsTransactionAndMappingCounts()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new MerchantService(context);
+
+        var account = new Account
+        {
+            Id = 1,
+            Institution = "Test Bank",
+            AccountType = AccountType.Checking,
+            Last4 = "1234",
+            Owner = "Test Owner"
+        };
+        context.Accounts.Add(account);
+
+        var merchant = new Merchant { Id = 1, Name = "Walmart" };
+        context.Merchants.Add(merchant);
+
+        var transaction = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -50.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            MerchantId = 1
+        };
+        context.Transactions.Add(transaction);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.DeleteMerchantAsync(1);
+
+        // Assert
+        Assert.True(result.Success);
+        Assert.Equal(1, result.TransactionCount);
+        Assert.Contains("1 transaction", result.Message);
+    }
+}