fix:restored removed article to available articles

[shishiro26]

NOTE: Please review the pull request process before opening your first PR: https://github.com/WikiEducationFoundation/WikiEduDashboard/blob/master/CONTRIBUTING.md#pull-request-process

What this PR does

This PR restores both self-assigned and admin-assigned articles back to the pool of available articles.
I have written a test case for the controller to ensure that the changes work as expected.

Screenshots

After:
https://github.com/user-attachments/assets/db77ecb3-734d-4693-94d2-cfd235297c90

Open questions and concerns

With this change, when removing an article, a dialog appears with the message: "Are you sure you want to delete this program?"

• Do we need to modify the dialog message to reflect the new functionality?

#5549

[shishiro26]

In this PR, I have added Ruby test cases for the unclaim method in the AssignmentsController.

The scenarios covered are as follows:

1. When the article is assigned to the user by the admin:
   • The admin removes the assignment.
   • https://github.com/user-attachments/assets/7bbd0d4e-bac7-4f56-9066-af09b595dc84
2. When the article is assigned to the user by the admin:
   • The user removes the assignment.
   • https://github.com/user-attachments/assets/392911db-789f-44dc-81c7-84f73232d7a6
3. When the article is self-assigned:
   • Either the user or the admin removes the self-assigned article.
   • https://github.com/user-attachments/assets/ea029f97-58a5-455c-996d-4d46ee9ee9a2

[shishiro26]

@ragesoss the test is passing locally in my system