Add SEP-10 JWT refresh before expiry in long-running flows

[Haroldwonder]

sep10_jwt.rs validates tokens but does not proactively refresh them. Add a refresh_if_expiring(token, threshold_secs) helper that re-authenticates before the JWT window closes.

[thaiwoz02]

@thaiwoz02 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer, I would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @thaiwoz02 to this issue.

[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, @micheamitchelle-rgb! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @micheamitchelle-rgb: 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.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊