Improve unit test coverage for User registration layer

[zigzagdev]

Overview

Add unit tests for the uncovered classes in the User registration feature to meet the Codecov patch coverage target.

Target files

• Subscription — isActive(), isExpired(), isFree()
• CreateUserCommand — fromArray() key validation
• CreateUserUseCase — handle()
• UserViewModel — toArray()
• UserDtoFactory — build()

Parent Issue

Part of #400