Implement unit test for each URL access

[hwakabh]

AsIs

In #50, and also #55, we have implemented baseline of unit testings for the app with push-trigger pipeline, but still need to implement other test cases for application behaviors.
At least, we have to check with API routes, since they are accessed from frontend JavaScript, script.js.

ToDo

• Research best practices for testings with FastAPI. Meaning, make it clear how we should separate test cases as files within tests/ directory
• Implement test cases for checking behaviors of API endpoints, which are defined in app/api/v1/routes.py

(Optional) Acceptance Criteria

Test pipeline will be passed