[php-src] Issue #21299: php-fpm: php_admin_* do not overwrite php.ini , only if the value is commented out in php.ini

Issue: https://github.com/php/php-src/issues/21299
Author: Cyborgscode

### Description

Name        : php-fpm
Version     : 8.4.18


The system once was intended to override global php.ini settings with pool specific values, in special for memory management. 

Actual situation: 

global php.ini overrides pool directives:

# grep memory_limit /etc/php.ini
memory_limit = 512M

# grep memory_limit /etc/php-fpm.d/pools.conf 
php_admin_value[memory_limit] = 128MB
php_admin_value[memory_limit] = 128MB
php_admin_value[memory_limit] = 128MB
php_admin_value[memory_limit] = 128MB

but ..  ini_get('memory_limit') inside the php script shows 512M, instead of 128MB.

This MUST BE a mistake, because this way around does not make any sense.

ATM: to get 128MB set, you have to outcomment memory_limit in php.ini

The intended way must have been:

1. Read in php.ini
2. Read in pool.conf
2a. if given, override given setting.

Logic dictates, that with more than one pool.conf it's not ensured, that all pools set a memory_limit override. That said, outcommenting the global php.ini entry can't be the solution, because this would lead directly to an undefined state ( in fact it won't because hardcoded emergc defaults in php.exe prevent this). 

If that ever worked as intended, someone forgot the build unittests for this. 

Docs say this:

PHP settings passed with php_value or php_flag will overwrite their previous value. Please note that defining disable_functions or disable_classes will not overwrite previously defined php.ini values, but will append the new value instead.

Settings defined with php_admin_value and php_admin_flag cannot be overridden with ini_set().
--

Hard to say if it's a regression.

Reproducible: Always

Steps to Reproduce:
1. create a pool with a given memory limit
2. use the php default php.ini
3. restart php-fpm
4. check memory_limit in info.php 

=> global limit set.

Actual Results:          use of global limits, in case they are globally defined.
Expected Results:     use of overwritten limits.

### PHP Version

```plain
PHP 8.4.18 (fpm-fcgi) (built: Feb 10 2026 17:48:03) (NTS gcc x86_64)
Copyright (c) The PHP Group
Built by Fedora Project
Zend Engine v4.4.18, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.18, Copyright (c), by Zend Technologies
```

### Operating System

Fedora 42