[php-src] Issue #20727: Default mbstring detect order does not work in header_register_callback()

Issue: https://github.com/php/php-src/issues/20727
Author: nschilling

### Description

The following code:

```php
<?php
header_register_callback( fn() => mb_detect_encoding('foo') );
```

Resulted in this output:
```
Fatal error: Uncaught ValueError: mb_detect_encoding(): Argument #2 ($encodings) must specify at least one encoding in /in/0EDPU:2
Stack trace:
#0 /in/0EDPU(2): mb_detect_encoding('foo')
#1 [internal function]: {closure:/in/0EDPU:2}()
#2 {main}
  thrown in /in/0EDPU on line 2
```

But I expected this output instead:
```
```

The error seems to be present since PHP 8.0.0
https://3v4l.org/0EDPU

The error also occurs when `mb_convert_encoding()` is called instead of `mb_detect_encoding()`.

# Workaround
Setting `mbstring.detect_order` explicitly in `php.ini` prevents the error:
```
; php.ini
mbstring.detect_order = "ASCII,UTF-8";
```

Setting the order with `mb_detect_order()` inside the callback also prevents the issue:
```
header_register_callback( function() {
    mb_detect_order(['ASCII', 'UTF-8']);
    mb_detect_encoding('foo');
} );
```

Setting the order outside of the callback or setting it with `ini_set()` did **not** prevent the error.

### PHP Version

```plain
PHP 8.4.15 (cli) (built: Dec  8 2025 22:51:21) (NTS)
Copyright (c) The PHP Group
Built by https://github.com/docker-library/php
Zend Engine v4.4.15, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.15, Copyright (c), by Zend Technologies
    with Xdebug v3.5.0, Copyright (c) 2002-2025, by Derick Rethans
```

### Operating System

_No response_