Write integration tests for SEP-6 deposit happy path

[Haroldwonder]

sep6.rs has limited test coverage for the full deposit flow. Add integration tests in src/sep6_integration_tests.rs covering: initiate deposit → poll status → confirm completion.

[Nabeelahh]

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

Hi maintainer I have thoroughly read the description to this issue and I can resolve this according to the description required ensuring it meet all the acceptance criteria kindly assign.

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

[drips-wave]

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

🧑‍💻 @john-augustine-create: 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.

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