fix(authn): fix pbkdf2 option validation

[savonarola]

Fixes EMQX-10777

Also fixed a bug with password hashing made inside a transaction.

Summary

🤖 Generated by Copilot at 364b3e2

This pull request fixes a bug and improves the validation and testing of the PBKDF2 hashing algorithm for password authentication. It changes the schema of the iterations parameter to pos_integer(), adds a guard clause to the hash function, and adds a test case for the schema validation.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• [na] Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Schema changes are NOT backward compatible. However, configurations that are now forbidden couldn't be functional.