Increase Test Coverage for Edge Cases

[Mkalbani]

Acceptance Criteria

• Add tests for expired quests, duplicate joins, zero rewards, and unauthorized actions.
• Coverage should significantly improve.
• CI pipeline must pass successfully.

[gerani1]

@gerani1 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hello, working on this will be fast, can i get assigned

ℹ️ Repo Maintainers: To accept this application, review their application or assign @gerani1 to this issue.