feat(application): commands/queries + IProductPricingService (PRD-003)

- IProductPriceRepository (AddAsync/GetByProductIdAsync/GetActiveAsync)
- ProductPriceDto, AddProductPriceCommand/Response, GetProductPricesQuery
- AddProductPriceCommandValidator (FluentValidation + TimeProvider, fecha >= hoy_AR)
- AddProductPriceCommandHandler (TransactionScope AsyncFlow, audit fail-closed)
- GetProductPricesQueryHandler (verifica producto existe, lista vacía válida)
- IProductPricingService + ProductPricingService (GetPriceAtAsync → decimal?)
- DI wiring en DependencyInjection.cs
- 29 tests NSubstitute + FakeTimeProvider, 1081 Application.Tests GREEN

tests/SIGCM2.Application.Tests/Products/Prices/GetProductPricesQueryHandlerTests.cs

@@ -0,0 +1,113 @@
+using FluentAssertions;
+using NSubstitute;
+using SIGCM2.Application.Abstractions.Persistence;
+using SIGCM2.Application.Products.Prices;
+using SIGCM2.Application.Products.Prices.GetHistory;
+using SIGCM2.Domain.Entities;
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Application.Tests.Products.Prices;
+
+/// <summary>
+/// PRD-003 — GetProductPricesQueryHandler tests.
+/// Covers: §REQ-4.1 (historial descending), §REQ-4.3 (lista vacía), §REQ-3.3 (producto no existe → 404).
+/// </summary>
+public class GetProductPricesQueryHandlerTests
+{
+    private readonly IProductPriceRepository _pricesRepo = Substitute.For<IProductPriceRepository>();
+    private readonly IProductRepository _productsRepo = Substitute.For<IProductRepository>();
+    private readonly GetProductPricesQueryHandler _handler;
+
+    private static readonly DateOnly Date1 = new(2026, 1, 1);
+    private static readonly DateOnly Date2 = new(2026, 2, 1);
+    private static readonly DateOnly Date3 = new(2026, 3, 1);
+
+    private static Product ActiveProduct() => Product.ForCreation(
+        "Test", medioId: 1, productTypeId: 2,
+        rubroId: null, basePrice: 100m, priceDurationDays: null,
+        TimeProvider.System);
+
+    private static ProductPrice MakePrice(long id, DateOnly pvf, DateOnly? pvt = null) =>
+        new(id, ProductId: 1, Price: 100m * id, pvf, pvt,
+            FechaCreacion: new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc));
+
+    public GetProductPricesQueryHandlerTests()
+    {
+        _productsRepo.GetByIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(ActiveProduct());
+
+        // default: lista con 2 precios, el repo ya los devuelve descending (responsabilidad del repo)
+        _pricesRepo.GetByProductIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<ProductPrice>
+            {
+                MakePrice(3, Date3),          // activo (pvt=null)
+                MakePrice(2, Date2, Date3.AddDays(-1)), // cerrado
+                MakePrice(1, Date1, Date2.AddDays(-1))  // cerrado más antiguo
+            }.AsReadOnly() as IReadOnlyList<ProductPrice>);
+
+        _handler = new GetProductPricesQueryHandler(_pricesRepo, _productsRepo);
+    }
+
+    // ── Orden descending ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Handle_ReturnsAllPrices_InDescendingOrder()
+    {
+        // §REQ-4.1 — historial completo ordenado descending por PriceValidFrom
+        var result = await _handler.Handle(new GetProductPricesQuery(ProductId: 1));
+
+        result.Should().HaveCount(3);
+        result[0].PriceValidFrom.Should().Be(Date3);  // más reciente primero
+        result[1].PriceValidFrom.Should().Be(Date2);
+        result[2].PriceValidFrom.Should().Be(Date1);
+    }
+
+    [Fact]
+    public async Task Handle_MapsToDto_WithIsActive()
+    {
+        var result = await _handler.Handle(new GetProductPricesQuery(ProductId: 1));
+
+        result[0].IsActive.Should().BeTrue();   // pvt=null → activo
+        result[1].IsActive.Should().BeFalse();  // pvt IS NOT NULL → cerrado
+    }
+
+    // ── Lista vacía (nuevo producto sin precios) ─────────────────────────────────
+
+    [Fact]
+    public async Task Handle_EmptyHistory_ReturnsEmptyList()
+    {
+        // §REQ-4.3 — nuevo producto aún no tiene precios → lista vacía (no 404)
+        _pricesRepo.GetByProductIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<ProductPrice>().AsReadOnly() as IReadOnlyList<ProductPrice>);
+
+        var result = await _handler.Handle(new GetProductPricesQuery(ProductId: 1));
+
+        result.Should().BeEmpty();
+    }
+
+    // ── Producto inexistente ────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Handle_ProductNotFound_ThrowsProductNotFoundException()
+    {
+        _productsRepo.GetByIdAsync(99, Arg.Any<CancellationToken>())
+            .Returns((Product?)null);
+
+        var act = async () => await _handler.Handle(new GetProductPricesQuery(ProductId: 99));
+
+        await act.Should().ThrowAsync<ProductNotFoundException>();
+    }
+
+    [Fact]
+    public async Task Handle_ProductNotFound_RepoGetByProductId_NotCalled()
+    {
+        _productsRepo.GetByIdAsync(99, Arg.Any<CancellationToken>())
+            .Returns((Product?)null);
+
+        var act = async () => await _handler.Handle(new GetProductPricesQuery(ProductId: 99));
+        await act.Should().ThrowAsync<ProductNotFoundException>();
+
+        await _pricesRepo.DidNotReceive()
+            .GetByProductIdAsync(99, Arg.Any<CancellationToken>());
+    }
+}