Add contract testnet integration test suite

[Haroldwonder]

Overview\nThere are unit tests and testnet scripts but no automated integration test suite that runs all contract functions against Stellar testnet.\n\n## Tasks\n- [ ] Create tests/integration/ directory in contract crate\n- [ ] Write integration tests: create, accept, confirm, cancel, dispute, expire\n- [ ] Run with cargo test --features testnet-integration\n- [ ] Add to CI nightly job\n\n## Acceptance Criteria\n- All major contract flows pass automated integration tests against Stellar testnet

[Dydex]

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

Hi Maintainer, Pls I can work on this issue

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[koder-ns]

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

hi, I am a good develper with problem solving and debugging skills. and an enthusiastic when it comes to open source. i would like to address this issue if assigned thank you

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[wendypetersondev]

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

Hi i would like to apply for this issue

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[drips-wave]

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

🧑‍💻 @wendypetersondev: 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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