[Chore] Increase test coverage

[malparty]

Issue

Automated tests do not cover the core business logic (CSV upload and the keywords retrieval).

Expected

While 100% test coverage is not required for this code challenge, all critical paths of the application should be unit-tested (and, ideally, UI-tested). So the following should be covered:

• CSV parsing
• Keyword creation
• Keyword scraping
• Keyword listing

Note There is a bit of a challenge to implement tests for a service that generates HTTP requests. Hence, it is an expected part of the submission for us to assess your skills in writing challenging tests.

[anhdt1911]

Noted! Will add missing test cases for the core business logic.