[CONTRACT] Add regression tests for settlement state machine transitions

[dinahmaccodes]

Tier: Hard

Description: Extend settlement tests so the pending/completed/failed lifecycle stays locked to the actual token-transfer behavior.

Tasks:

• Add a test that walks a settlement from pending to completed
• Add a failure-path test that records an error code
• Assert the active settlement link is cleared

Acceptance Criteria:

• Pending settlements become completed exactly once
• Failure stores an error code and clears active state
• Settlement state updates are deterministic

Files: contracts/shipment/src/test_settlement_machine.rs, contracts/shipment/src/test_settlement.rs

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[Yunusabdul38]

@Yunusabdul38 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i work on this

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[drips-wave]

Congratulations, @Yunusabdul38! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Yunusabdul38: 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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[jadonamite]

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

Hi maintainer can i work on this and get it done ASAP

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[Fayvor22]

@Fayvor22 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would love to work on this issue and resolve it. Please assign

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[OtowoSamuel]

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

I would love to work on this!

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[drips-wave]

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

🧑‍💻 @OtowoSamuel: 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 🌊