[php-src] Issue #21338: metadata checks (chatsetnr) in various mysqli tests fail

Issue: https://github.com/php/php-src/issues/21338
Author: 9EOR9

### Description

The following test:

```sapi/cli/php -f run-tests.php ext/mysqli/tests/mysqli_fetch_field.phpt
```

Resulted in this output:
```========DIFF========
--
       ["decimals"]=>
       int(0)
     }
057+ [004] Expecting charset utf8/33 got 192
     bool(false)
     mysqli_result object is already closed
     array(1) {
--
========DONE========
FAIL mysqli_fetch_field() [ext/mysqli/tests/mysqli_fetch_field.phpt] 
=====================================================================
Number of tests :     1                 1
Tests skipped   :     0 (  0.0%) --------
Tests warned    :     0 (  0.0%) (  0.0%)
Tests failed    :     1 (100.0%) (100.0%)
Tests passed    :     0 (  0.0%) (  0.0%)
---------------------------------------------------------------------
Time taken      : 0.110 seconds
=====================================================================

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
mysqli_fetch_field() [ext/mysqli/tests/mysqli_fetch_field.phpt]
=====================================================================

```
mysqli_set_charset() selects a character set by name only.
However, in MySQL/MariaDB a character set can have multiple collations, and the actual collation used determines the internal character set number (charsetnr) returned in metadata.

Several tests in the ext/mysqli test suite assume that setting utf8mb3 results in charsetnr = 33 (which corresponds to utf8mb3_general_ci). This assumption is not valid when the server default collation differs.

As a result, multiple tests fail depending on server configuration.

**Possible workarounds (quick fix):**
- use %d in the expect section
- use SET NAMES utf8mb3 COLLATE 'utf8mb3_general_ci' if you want to explicitly test for this character set/collation.

**Long term solution:**
Implement session_tracking (CLIENT_SESSION_TRACK capability) and obtain the character set information from session tracking information part of OK packet  (or if not changed from server greeting packet),

### PHP Version

```plain
PHP 8.3.6 (cli) (built: Jan  7 2026 08:40:32) (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

_No response_