Feature/user service tests

[stark-cybertech]

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Code refactor
• 📝 Documentation update
• 🚀 Performance improvement
• ✅ Test addition/update
• 🧹 Chore/maintenance
• 📐 API Spec update
• Other (please describe):

Changes

• add junit tests to user-microservice
• improve input validation and exception handling

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the API spec to reflect any changes my code makes

Related Issues

Fixes #
Related to #

Additional Notes