fix: return 400 for duplicate username or email (#61)

[30nap]

Description

This PR fixes an issue where registering a user with an existing username or email address returned 500 Internal Server Error instead of 400 Bad Request.

The exception handling has been updated to return the appropriate HTTP status code for duplicate username or email validation errors.

Fixes #61

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Tested user registration with an already existing username.
• Tested user registration with an already existing email address.
• Verified that both cases return 400 Bad Request instead of 500 Internal Server Error.
• Ran the existing unit tests to ensure no regressions were introduced.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• New and existing unit tests pass locally with my changes

[github-actions]

Thanks so much @30nap for creating your first PR, the Spring-Boot-Blog-REST-API community thanks you :)