refactor: reduce_multiples() and fix user guide

[kenjis]

Description

• refactor to suppress PHPStan error (false positive)
• fix user guide
• refactor test code

 ------ ------------------------------------------------------------------- 
  Line   system/Helpers/text_helper.php                                     
 ------ ------------------------------------------------------------------- 
  529    Regex pattern is invalid: Compilation failed: quantifier does not  
         follow a repeatable item at offset 5 in pattern: #.*{2,}#          
         🪪  regexp.pattern                                                 
 ------ ------------------------------------------------------------------- 

https://github.com/codeigniter4/CodeIgniter4/actions/runs/10208178324/job/28244217081

But the pattern is #,{2,}# by default:

CodeIgniter4/system/Helpers/text_helper.php

Lines 527 to 529 in 1d3336b

	function reduce_multiples(string $str, string $character = ',', bool $trim = false): string
	{
	$str = preg_replace('#' . preg_quote($character, '#') . '{2,}#', $character, $str);

I sent a bug report: phpstan/phpstan#11432

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[paulbalandan]

This seems to be a bug in PHPStan.