[php-src] Issue #22279: mb_check_encoding doesn't respect UTF-8 RFC3629

Issue: https://github.com/php/php-src/issues/22279
Author: LLyaudet

### Description

Hello,

RFC3629 says:
```
   The definition of UTF-8 prohibits encoding character numbers between
   U+D800 and U+DFFF, which are reserved for use with the UTF-16
   encoding form (as surrogate pairs) and do not directly represent
   characters.
```

But both of mb_check_encoding and preg_match('//u', $input) doesn't detect when such characters occurs.
I found this whilst validating my code here:
https://github.com/Seldaek/jsonlint/pull/91

I had to comment out the fast path relying on mb_check_encoding and preg_match.

Maybe there are related security issues.

Have a nice day, best regards,
    Laurent Lyaudet


### PHP Version

```plain
PHP 8.5.4 (cli) (built: May 25 2026 12:19:37) (NTS)
Copyright (c) The PHP Group
Built by Ubuntu
Zend Engine v4.5.4, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.4, Copyright (c), by Zend Technologies
```

### Operating System

Ubuntu 26.04