[CT-16] Implement cross-contract tokenization call pattern in opsce crate

[yusuftomilola]

Problem

The assetsup and multisig_transfer contracts operate independently with no cross-contract communication. A tokenized asset transfer approved by the multisig wallet cannot automatically invoke transfer_tokens() on the assetsup contract.

Proposed Solution

Create contracts/opsce/src/cross_contract.rs. Implement a client interface that allows multisig_transfer to call assetsups transfer_tokens() via Soroban cross-contract invocation.

Acceptance Criteria

• A AssetsUpClient struct defined using Sorobans contractclient macro targeting the assetsup contract interface
• multisig_execute_token_transfer(env, wallet_id, tx_id, assetsup_contract_id) invokes transfer_tokens on the assetsup contract after multisig approval
• Correct authorization (require_auth) passed through the cross-contract call
• If the cross-contract call fails, the entire multisig execution reverts
• The assetsup_contract_id is stored in the multisig wallet config and changeable by the wallet admin
• Unit tests mock the cross-contract call using Soroban test utilities

[Alimzy]

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

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.
ETA: 1-3 days (depending on scope)

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

[ummarig]

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

Can I please be assigned this task

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

[drips-wave]

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

🧑‍💻 @ummarig: 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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