Add SEP-6 KYC status polling helper

[Haroldwonder]

After initiating a deposit, callers must poll manually for KYC completion. Add a poll_kyc_status(transaction_id, interval_secs, max_attempts) helper in sep6.rs that uses the retry module.

[EmannuellaO]

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

Hi! I've carefully reviewed the entire task description and fully understand the requirements and expected outcome.
I have experience working on similar tasks and I'm confident I can complete this within about an hour. I'd be happy to take ownership of this issue and submit a clean, well-tested solution promptly.
If it's available, please assign it to me. Thank you!

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[Olisachukwuma1]

@Olisachukwuma1 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Good morning sir please I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Olisachukwuma1 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.

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