fix: Dont use mb_detect_encoding for mb_string function calls

[DannyvdSluijs]

Description

Avoid mb_detect_encoding for draft 3 and 4 validations

Related Issue

Fixes #923

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Other (please describe):

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the code style of this project
• I have added tests that prove my fix is effective or that my feature works
• All new and existing tests pass
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional Notes

All mb_string function calls use the default value null which results in the internal character encoding value being used which defaults to UTF-8 and allows for local overrides through setup.

[Copilot]

Pull request overview

This PR updates Draft 3/4 string length validation to avoid mb_detect_encoding() during minLength/maxLength checks, addressing the nondeterminism reported in #923.

Changes:

• Remove mb_detect_encoding() usage from StringConstraint::strlen(), relying on mb_strlen() instead.
• Drop the now-obsolete PHPStan baseline entry related to mb_strlen() being called with a string|false encoding.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/JsonSchema/Constraints/StringConstraint.php	Removes mb_detect_encoding() from string length calculation for Draft 3/4 validation.
phpstan-baseline.neon	Removes the baseline suppression that was only needed due to the previous mb_detect_encoding() return type.

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