fix: prevent password length validation fatal

[Yehonal]

Changes

• Qualifies the password length constant as \ACore\Manager\UserValidator::PASSWORD_LENGTH during WordPress password validation.
• Updates the validation message wording while keeping the same 16-character limit.

Why

When a submitted password is longer than the AzerothCore limit, the validator tried to read an unqualified PASSWORD_LENGTH constant. In PHP 8 this can fatal instead of returning a validation error to WordPress.

Testing

• Passed: git diff --check
• Not run: php -l, composer tests; php/composer/docker are not installed in this runtime.