[Client] Add Unit Tests for Frontend Services

[GravityDarkLab]

📝 Description

Implement unit and integration tests for all service modules under the services/ directory in the SkillForge frontend. These tests should mock API interactions where appropriate and ensure that services correctly communicate with backend endpoints.

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✅ Checklist

• achievement.service.ts tested
• aiChat.service.ts tested
• course.service.ts tested
• dashboard.service.ts tested
• user.service.ts tested
• API calls are properly mocked (e.g., using MSW or Jest mocks)
• Tests run successfully in the CI pipeline

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🎯 Goal

Ensure client services reliably handle backend communication logic and behave correctly under test conditions, improving maintainability and stability of the frontend application.