Add `get_invoice_deadline` view function returning deadline timestamp

[Kingsman-99]

Description

Invoice deadlines are key for frontend expiry countdown UI. There is no standalone getter for an invoice's deadline field. Adding get_invoice_deadline(invoice_id: u64) -> u64 avoids forcing callers to decode the full struct.

Acceptance Criteria

• Add pub fn get_invoice_deadline(env: Env, invoice_id: u64) -> u64 to contracts/split/src/lib.rs
• Reads deadline from InvoiceCore persistent storage
• Returns ContractError::InvoiceNotFound for unknown IDs
• Unit test verifies the returned value matches the deadline set at creation
• cargo test passes

Context

• Target file: contracts/split/src/lib.rs

[RemmyAcee]

@RemmyAcee has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I've checked the requirements and am confident I can resolve this efficiently. Happy to take it on.

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

[fejilaup-cloud]

@fejilaup-cloud has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Please can I get assigned to contribute to this please

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

[drips-wave]

Congratulations, @fejilaup-cloud! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @fejilaup-cloud: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@fejilaup-cloud's PR #643 was approved and merged by @Kingsman-99.

🏆 @fejilaup-cloud: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (236 total points). Track your full progress on GrantFox.

👏 Great work, @fejilaup-cloud! Keep contributing to Stellar-split.

[drips-wave]

This issue has been completed by @fejilaup-cloud as part of the Stellar Wave Program's 8th Wave 🥳

😎 @fejilaup-cloud: 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.