[Bug] PHP 8.5 has mbstring compiled in, but mbstring.sh appends mbstring.so and causes "Module already loaded"

Environment:

• Ubuntu 26.04 LTS x86_64
• PHP 8.5.8 installed by aaPanel
• Script: /www/server/panel/install/mbstring.sh

Problem:
aaPanel builds PHP 8.5 with --enable-mbstring, so mbstring is already
available even when PHP is started with -n.

Reproduction:

/www/server/php/85/bin/php -n -r
'echo extension_loaded("mbstring") ? "loaded\n" : "missing\n";'

Output:
loaded

After installing mbstring from aaPanel, mbstring.sh appends:

extension = /www/server/php/85/lib/php/extensions/no-debug-non-zts-20250925/mbstring.so

to both php.ini and php-cli.ini.

Every PHP invocation then prints:

PHP Warning: Module "mbstring" is already loaded in Unknown on line 0

Root cause:
Install_mbstring() only treats mbstring as installed when both a
/mbstring.so configuration line and php -m output exist. It does not
detect a statically compiled mbstring module.

Suggested fix:
Before building or appending mbstring.so, test the no-configuration PHP
binary:

/www/server/php/$version/bin/php -n -r
'exit(extension_loaded("mbstring") ? 0 : 1);'

If it returns 0, report mbstring as built in and do not append the
dynamic extension line.

Start with /www/server/panel/install/mbstring.sh and inspect Install_mbstring(). Run the provided php -n extension_loaded check alongside the existing installation checks. Done means built-in mbstring is recognized without appending mbstring.so to php.ini or php-cli.ini, and PHP invocations no longer warn.