Add comprehensive tests for User schema and JWT authentication utilities

[AdithaBuwaneka]

• Implement tests for User model including password hashing, comparison, and schema validation.
• Mock bcrypt and mongoose for isolated testing of password-related functionalities.
• Validate password handling in various scenarios including error cases.
• Add tests for JWT authentication utilities to ensure proper token validation and extraction.
• Mock jsonwebtoken to simulate different JWT scenarios and edge cases.
• Ensure environment variable checks and error handling are covered in tests.