[#14] Use static Mockito mock imports in command-service tests

[igorsatsyuk]

Summary

• replace Mockito.mock(...) with static mock(...) imports in command-service tests
• remove now-unused Mockito imports reported by SonarQube
• keep test behavior unchanged while matching the repository Mockito style

Testing

• mvn -B -pl command-service -am test -Dtest=AdditionalCommandServiceTest,UserLoginCommandServiceTest -Dsurefire.failIfNoSpecifiedTests=false

Closes #14