PR Title:
feat: prevent duplicate reward claims and add security tracking (#206)
PR Description:
## Description
Resolves #206.
This PR introduces an important security layer to prevent multiple reward claims by the same participant, effectively protecting the reward system from exploitation. Reward claims are now persistently tracked per participant, and any subsequent claim attempts will safely revert.
## Changes Made
- Added secure state-tracking logic to record claimed rewards per participant address.
- Implemented validation checks in the claim flow to ensure repeated claim attempts revert with an appropriate error.
- Added comprehensive regression tests to verify both successful claims, duplicate claims, and edge cases.
- Ensured the protection logic integrates cleanly without affecting existing reward flows.
## Acceptance Criteria Met
- [x] Claimed rewards are explicitly tracked per participant.
- [x] Multiple claim attempts correctly revert.
- [x] Regression tests added to verify protection logic.