Add Quest Expiry Timestamp Validation

[Mkalbani]

Description

Quests with deadlines have no on-chain enforcement. A player can complete an expired quest and still claim rewards.

Expected Behavior

The contract should revert if a player attempts to complete a quest past its expiry time.

Proposed Fix

Add a timestamp guard in the completion function:

require(block.timestamp < quest.expiresAt, "Quest: expired");

Acceptance Criteria

• Expired quests cannot be completed
• Test cases cover boundary conditions (exactly at expiry)
• Event emitted on expiry rejection

[JoaquinSkorg]

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

Hello Maintainer, I'd like to work on this issue. Kindly assign it to me

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

[drips-wave]

Congratulations, @JoaquinSkorg! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @JoaquinSkorg: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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