[QuestEngine] Prevent duplicate Explore quest verification payouts

[Kaylahray]

Description

The Explore Quest verification flow should track whether a learner has already been paid for a specific Explore Quest, preventing accidental or malicious repeated admin-triggered payouts.

User Story

As the Learnault Protocol,
I want to ensure each Explore Quest payout is one-time per learner,
so that admin retries or replayed calls cannot duplicate reward distribution.

Requirements and Context

• Files: contracts/quest-engine/src/lib.rs, contracts/quest-engine/src/types.rs, contracts/quest-engine/src/test.rs
• Add storage that records Explore Quest verification per (learner, quest_id)
• verify_explore_quest must revert if the learner was already verified for that quest
• Preserve the current admin-only and QuestType::Explore checks

Suggested Implementation

// 1. Add a verification DataKey for (learner, quest_id).
// 2. Assert the key does not already exist before payout.
// 3. Call RewardPoolClient::distribute_reward.
// 4. Persist the verification marker after successful payout.

Acceptance Criteria

• A learner can only be paid once per Explore Quest.
• Repeated verification attempts revert cleanly.
• Different learners on the same Explore Quest remain independently payable.

Submission Guidelines

• Branch: feat/explore-verification-replay-guard
• Depends on: [QuestEngine] Verify explore quest
• PR: fix(quest-engine): prevent duplicate explore quest payouts

[Silver36-ship-it]

I would love to work on this professionally, please assign it to me

[Martins-594]

Hi, I'd like to work on this issue. I've gone through the description and acceptance criteria and I'm confident I can deliver it. I'll make sure every acceptance criterion is met and submit a clean, tested PR. Please assign it to me.

[syed-ghufran-hassan]

Please assign this issue to me as below is the issue and resolution:

Issue: No storage check allowed repeated verify_explore_quest calls to pay the same learner multiple times;

Fix: I will Add an ExploreVerification(Address, u32) marker that is checked before payout and persisted after, ensuring one‑time payment per (learner, quest_id).

[grantfox-oss]

🦊 GrantFox — @syed-ghufran-hassan has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #91)
2. Your PR will be reviewed by the learnault maintainers

Good luck! Track your progress on GrantFox.

[syed-ghufran-hassan]

@Kaylahray I have raised following PRs please check

#97
#102