Add basic backend tests for challenges app

[iceybubble]

Description
The apps/challenges module contains core challenge-related logic (views, serializers, permissions), but currently does not have a dedicated tests directory.

Why this matters
Lack of automated tests makes it harder to prevent regressions in challenge creation, listing, and permission handling.

Proposed solution
Introduce a tests module under apps/challenges and add basic backend tests covering:

• Challenge list API access
• Permission checks (unauthenticated vs authenticated users)
• Basic serializer validation

Scope
Tests only. No functional changes.

[iceybubble]

I’d like to work on this issue and submit a PR.

[RitikaSen27]

hi i want to contribute in this issue.
I have submitted a PR addressing this issue by adding a serializer validation test for ChallengeSerializer. Please let me know if any changes are needed.