[CT-19] Write comprehensive tests for multisig-wallet contract in opsce crate

[yusuftomilola]

Problem

contracts/multisig-wallet/src/tests.rs exists but coverage is minimal. The approval threshold logic, owner management, and transaction lifecycle (submit, approve, execute) are not fully tested.

Proposed Solution

Create contracts/opsce/src/multisig_tests.rs with a complete test suite for the multisig wallet using soroban_sdk::testutils.

Acceptance Criteria

• initialize() — correct threshold set, duplicate init fails
• submit_transaction() — success, non-owner submission fails
• approve_transaction() — approvals accumulate, duplicate approval by same owner is rejected
• execute_transaction() — executes at threshold, fails below threshold, fails on re-execution
• revoke_approval() — revocation reduces count, execution fails after sufficient revocations
• add_owner() and remove_owner() — non-owner calls fail; removing last owner fails
• 2-of-3 and 3-of-5 threshold scenarios both tested
• Tests run cleanly with cargo test -p multisig-wallet

[nafsonig]

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

Let me implement this task as soon as possible

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

[drips-wave]

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

🧑‍💻 @nafsonig: 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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