refactor: eliminar NullProductQueryRepository dead code + EXISTS en ProductQueryRepository (PRD-002 S1 S2)

src/api/SIGCM2.Application/Products/NullProductQueryRepository.cs

@@ -1,14 +0,0 @@
-using SIGCM2.Application.Abstractions.Persistence;
-
-namespace SIGCM2.Application.Products;
-
-/// <summary>
-/// STUB — PRD-002 replaces the DI binding with a real Dapper impl against dbo.Product.
-/// Returns false for all queries so DeactivateProductTypeCommandHandler guard always passes.
-/// This is intentional for PRD-001: the mechanism is installed; the data feed arrives in PRD-002.
-/// </summary>
-public sealed class NullProductQueryRepository : IProductQueryRepository
-{
-    public Task<bool> ExistsActiveByProductTypeAsync(int productTypeId, CancellationToken ct = default)
-        => Task.FromResult(false);
-}