BACKEND - Create Tests for Username validation

[otvegg]

What

Implement unit and integration tests for the backend username validation logic. Tests should verify that all validation rules for usernames are correctly enforced and that both successful and failing cases are handled properly.

Why

• Ensure that username validation works according to specifications.
• Increase backend reliability and maintainability.

How

• Write unit tests for the Validator.Username (or equivalent) method to cover all validation rules.
• Write integration tests for the /validation/username endpoint.
• Cover both positive (valid usernames) and negative (invalid usernames) cases.
• Ensure descriptive test naming using the convention: