[CT-38] Add referral query functions

[yusuftomilola]

Overview

Add read-only query functions for referral history and reward status. Work inside contracts/sandbox/referral/.

Acceptance Criteria

• get_code(env, code) -> Result<ReferralCode, Error> returns code details
• get_referrer_code(env, referrer) -> Option<String> returns a referrer's own code
• get_referral(env, referee) -> Option<Referral> returns who referred a given address
• get_reward_amount(env) -> u128 returns the current configured reward
• Code compiles with cargo build

[xeeenab]

@xeeenab has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi, I’d like to work on implementing this feature and have reviewed the requirements. I’ll ensure it integrates cleanly with the existing codebase and follows the project’s guidelines, please let me know if there are any specific preferences before I begin.

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

[drips-wave]

Congratulations, @xeeenab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @xeeenab: 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.

Warning

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🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @xeeenab as part of the Stellar Wave Program's 4th Wave 🥳

😎 @xeeenab: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.