missing validation for unique name check in director search

## Description:

As a follow up to https://github.com/bcgov/entity/issues/3074 .
In the designs , there is a advance validation for the username where we validate the uniqueness of the username across the system.This ticket is to handle that part.
Also plz double check if there is anything missing from the designs other than this

Acceptance for a Task:
- [ ] Requires deployments
- [ ] Add/ maintain selectors for QA purposes
- [ ] Test coverage acceptable
- [ ] Linters passed
- [ ] Peer Reviewed
- [ ] PR Accepted
- [ ] Production burn in completed