[php-src] Issue #20674: mb_decode_mimeheader does not handle separator

Issue: https://github.com/php/php-src/issues/20674
Author: ewwink

### Description

if encoded Subject are too long they need to be separated the block by space `?=  =?` or line break, but mb_decode_mimeheader does not handle it correctly

The following code:

```php
<?php

$subject = '=?us-ascii?Q?The_PH?=  =?us-ascii?Q?P_8.5?=';
echo  mb_decode_mimeheader($subject) ;
```

PHP 8.3.21, 8.4.10 Result:
```
The PH  P 8.5
```

PHP  8.0.30, 7.4.33 Result:
```
The_PHP_8.5
```

But I expected this output instead, as `iconv_mime_decode` does:
```
The PHP 8.5
```


### PHP Version

```plain
8.3.21, 8.4.10
```

### Operating System

Ubuntu 24.04