feat(infrastructure): ProductQueryRepository.CountActiveByRubroAsync + integration test

Implementa SELECT COUNT(1) FROM dbo.Product WHERE RubroId = @RubroId AND IsActive = 1.
Tests de integración verifican: 0 sin productos, count correcto con mix
activos/inactivos/otro rubro, y solo inactivos retorna 0.

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

@@ -34,4 +34,16 @@ public sealed class ProductQueryRepository : IProductQueryRepository
         var result = await connection.ExecuteScalarAsync<int>(sql, new { ProductTypeId = productTypeId });
         return result == 1;
     }
+
+    public async Task<int> CountActiveByRubroAsync(int rubroId, CancellationToken ct = default)
+    {
+        const string sql = """
+            SELECT COUNT(1) FROM dbo.Product WHERE RubroId = @RubroId AND IsActive = 1
+            """;
+
+        await using var connection = _factory.CreateConnection();
+        await connection.OpenAsync(ct);
+
+        return await connection.ExecuteScalarAsync<int>(sql, new { RubroId = rubroId });
+    }
 }