@Override
    public Optional<Product> findProduct(int productId) {
        return this.productRepository.findById(productId);
    }
@Test
    void findProduct_ProductExists_ReturnsNotEmptyOptional() {
        // given
        var product = new Product(1, "Товар №1", "Описание товара №1");

        doReturn(Optional.of(product)).when(this.productRepository).findById(1);

        // when
        var result = this.service.findProduct(1);

        // then
        assertNotNull(result);
        assertTrue(result.isPresent());
        assertEquals(product, result.orElseThrow());

        verify(this.productRepository).findById(1);
        verifyNoMoreInteractions(this.productRepository);
    }