refactor: add assert() for PHPStan

[kenjis]

Description

Error: Binary operation "%" between string and non-empty-string&numeric-string results in an error.
 ------ ------------------------------------------------------ 
  Line   src/CronExpression.php                                
 ------ ------------------------------------------------------ 
  154    Binary operation "%" between string and               
         non-empty-string&numeric-string results in an error.  
 ------ ------------------------------------------------------ 

https://github.com/codeigniter4/tasks/actions/runs/10093662663/job/27909864651?pr=171

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

[coveralls]

Pull Request Test Coverage Report for Build 10105577918

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.676%

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Totals
Change from base Build 9986768704:	0.0%
Covered Lines:	336
Relevant Lines:	444

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💛 - Coveralls