Add string to Password Reset Username Rules

[spencerrlongg]

This is just a quick addition to the sendResetLinkEmail() method's validation rules to prevent attempting to submit an array, this wasn't getting through before, but it will no longer 500 when someone attempts this. I couldn't trigger the 500 using dot notation, it just seems like that gets prevented because it's an unused parameter name, doesn't seem like it comes through as an array.

[what-the-diff]

PR Summary

• Enhanced Validation Rules in sendResetLinkEmail Method
  The validation rules in the sendResetLinkEmail method have been updated to include checking if the 'username' field is a string. This enhancement aims to prevent potential issues that might occur when handling large data payloads.

[snipe]

Looks great, ty!