[php-src] Issue #21947: The flag 'JSON_UNESCAPED_LINE_TERMINATORS' seems to be useless

Issue: https://github.com/php/php-src/issues/21947
Author: JoeHorn

### Description

The following code:

```php
<?php
$strs = [
  'str1' => "Line1\nLine2",
  'str2' => "Line1\r\nLine2",
  'str3' => 'Line1' . PHP_EOL . 'Line2',
];
foreach ($strs as $k => $v) {
  echo "** {$k} **" . PHP_EOL . $v . PHP_EOL . PHP_EOL;
}

echo '** json_encoded **' . PHP_EOL . json_encode(
  $strs,
  JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_LINE_TERMINATORS
) . PHP_EOL . PHP_EOL;
```

Resulted in this output:
```
** str1 **
Line1
Line2

** str2 **
Line1
Line2

** str3 **
Line1
Line2

** json_encoded **
{"str1":"Line1\nLine2","str2":"Line1\r\nLine2","str3":"Line1\nLine2"}
```

But I expected this output instead:
```
** str1 **
Line1
Line2

** str2 **
Line1
Line2

** str3 **
Line1
Line2

** json_encoded **
{"str1":"Line1
Line2","str2":"Line1
Line2","str3":"Line1
Line2"}
```


### PHP Version

```plain
PHP 8.3.6 (cli) (built: Mar 20 2026 02:32:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.6, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.6, Copyright (c), by Zend Technologies
```

### Operating System

Ubuntu 24.04