[php-src] Issue #20619: Autoloader doesn't work in header_register_callback() callback when an exception is being thrown

Issue: https://github.com/php/php-src/issues/20619
Author: tgr

### Description

The following code:

```php
<?php

spl_autoload_register( function () {
        class Autoloaded {}
} );

header_register_callback( function () {
	new Autoloaded();
} );

throw new Exception();
```

Resulted in this output:
```
PHP Fatal error:  Uncaught Error: Class "Autoloaded" not found in /path/to/foo.php:8
Stack trace:
#0 [internal function]: {closure:/path/to/foo.php:7}()
#1 {main}
  thrown in /path/to/foo.php on line 8
```

But I expected this output instead:
```
Fatal error: Uncaught Exception in /path/to/foo.php:11
Stack trace:
#0 {main}
  thrown in /path/to/foo.php on line 11
```

Seen on Wikipedia servers first on PHP 7.4 then 8.1, reported on various other versions. Could reproduce locally on 8.5. Can't reproduce it on [3v4l.org](https://3v4l.org/lT1S9) for some reason.

### PHP Version

```plain
PHP 8.5.0 (cli) (built: Nov 20 2025 19:23:30) (NTS)
Copyright (c) The PHP Group
Built by Debian
Zend Engine v4.5.0, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.0, Copyright (c), by Zend Technologies
```

### Operating System

Ubuntu 24.04