xUnit: test isolation with IClassFixture and shared expensive setup

Creating expensive resources (database connections, WebApplicationFactory) in each test constructor is slow. Using static state across tests causes test pollution and flakiness.

Use IClassFixture<T> to share a single instance across all tests in a class:

// Fixture sets up expensive resource once
public class DatabaseFixture : IAsyncLifetime
{
    public AppDbContext Db { get; private set; } = null!;

    public async Task InitializeAsync()
    {
        var options = new DbContextOptionsBuilder<AppDbContext>()
            .UseInMemoryDatabase(Guid.NewGuid().ToString())
            .Options;
        Db = new AppDbContext(options);
        await Db.Database.EnsureCreatedAsync();
    }

    public async Task DisposeAsync() => await Db.DisposeAsync();
}

// Tests share the fixture
public class OrderTests : IClassFixture<DatabaseFixture>
{
    private readonly AppDbContext _db;

    public OrderTests(DatabaseFixture fixture)
        => _db = fixture.Db;

    [Fact]
    public async Task CreateOrder_Saves_ToDatabase()
    {
        var order = new Order { /* ... */ };
        _db.Orders.Add(order);
        await _db.SaveChangesAsync();
        Assert.Equal(1, await _db.Orders.CountAsync());
    }
}

xUnit creates a new test class instance per test method to ensure isolation. IClassFixture makes the fixture creation happen once per class and injects it. ICollectionFixture shares across multiple test classes.