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);
-}

src/api/SIGCM2.Infrastructure/Persistence/ProductQueryRepository.cs

@@ -19,16 +19,19 @@ public sealed class ProductQueryRepository : IProductQueryRepository
     public async Task<bool> ExistsActiveByProductTypeAsync(int productTypeId, CancellationToken ct = default)
     {
         const string sql = """
-            SELECT COUNT(1)
-            FROM dbo.Product
-            WHERE ProductTypeId = @ProductTypeId
-              AND IsActive = 1
+            SELECT CASE
+                WHEN EXISTS (
+                    SELECT 1 FROM dbo.Product
+                    WHERE ProductTypeId = @ProductTypeId
+                      AND IsActive = 1
+                ) THEN 1 ELSE 0
+            END
             """;
 
         await using var connection = _factory.CreateConnection();
         await connection.OpenAsync(ct);
 
-        var count = await connection.ExecuteScalarAsync<int>(sql, new { ProductTypeId = productTypeId });
-        return count > 0;
+        var result = await connection.ExecuteScalarAsync<int>(sql, new { ProductTypeId = productTypeId });
+        return result == 1;
     }
 }