[php-src] Issue #21616: DateTime::modify('-1 second') goes 59 minutes later on DST

Issue: https://github.com/php/php-src/issues/21616
Author: kylekatarnls

### Description

The following code:

```php
<?php
echo (new DateTime('2025-03-30 00:59:59', new DateTimeZone('Europe/London')))
    ->modify('+1 second')
    ->modify('-1 second')
    ->format('Y-m-d H:i:s.u e T');
```

Resulted in this output:
```
2025-03-30 02:59:59.000000 Europe/London BST
```

But I expected this output instead:
```
2025-03-30 00:59:59.000000 Europe/London GMT
```

Which is also the output we get when executing (since PHP 8.1, before that add/sub interval and modify gave the same output):
```php
echo (new DateTime('2025-03-30 00:59:59', new DateTimeZone('Europe/London')))
    ->add(new DateInterval('PT1S'))
    ->sub(new DateInterval('PT1S'))
    ->format('Y-m-d H:i:s.u e T');
```

### PHP Version

```plain
PHP 8.5.3
```

### Operating System

_No response_