Contracts: Add RWA shipment status transition guards

[Junirezz]

Context

YieldVault tracks tokenized RWA cargo via ShipmentStatus (Pending, InTransit, Delivered, Cancelled) with admin-only add_shipment and update_shipment_status.

Problem / Gap

update_shipment_status allows arbitrary transitions (e.g., Delivered → Pending) with no lifecycle rules. Cancelled shipments can be moved back to InTransit, undermining auditability of real-world asset backing.

Proposed approach

• Define an allowed transition matrix (e.g., Delivered and Cancelled are terminal).
• Return a typed error instead of silently mutating lists on invalid transitions.
• Add tests for each valid/invalid transition and list index consistency after updates.

Files/areas affected

• contracts/vault/src/lib.rs (update_shipment_status, ShipmentStatus)
• contracts/vault/src/test.rs (test_update_shipment_full_lifecycle_statuses)

[Yasir-TechGuy]

@Yasir-TechGuy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer,

I'd like to work on this issue because enforcing valid state transitions is essential for maintaining the integrity and auditability of tokenized real-world assets. I have experience with Rust, Soroban smart contracts, and designing state machines with strong validation and test coverage. I'm confident in implementing a clear transition matrix, typed errors, and comprehensive lifecycle tests that preserve shipment consistency.

Please assign this issue to me.

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

[Devsol-01]

@Devsol-01 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please can i work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Devsol-01 to this issue.

[drips-wave]

Congratulations, @Devsol-01! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Devsol-01: 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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