[BE] add unit tests to increase code coverage

[rot1024]

Target

• internal/adapter/gql/gqlmodel
• internal/infrastructure/mongo/mongodoc

Not target

• internal/infrastructure/* (needs mocking or running DBs) -> [BE] add mongo tests #14
• internal/adapter/* (low priority due to thin package)
• internal/usecase/* (refactoring in progress and possible code changes)
• pkg/* (not perfect, but generally well unit tested)

Policy

"Table-driven test" should be used only when test cases are diverse and It makes test codes more readable. It is not always necessary to use it.