Add unified 429 rate-limit error payload helper

[Chucks1093]

Summary
Add unified 429 rate-limit error payload helper

Scope

• standardize 429 response payload shape\n- include retry-after hint when available\n- apply helper across rate-limited routes

Acceptance Criteria

• scoped change is implemented with no unrelated file churn
• docs/tests are updated where applicable
• contributor can validate the outcome locally

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[Emmo00]

@Emmo00 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Emmo00 to this issue.

[logantalen]

@logantalen has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I'd liek to work on this

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

[drips-wave]

Congratulations, @logantalen! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @logantalen: 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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[drips-wave]

This issue has been completed by @logantalen as part of the Stellar Wave Program's 4th Wave 🥳

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