Add public response error code constants

[Chucks1093]

Summary

Add a shared set of error code constants for public endpoints.

Acceptance Criteria

• error codes are centralized and documented
• public error formatter can reference the constants
• change remains behavior-neutral

Contributor coordination: https://t.me/accesslayerorg

[SamixYasuke]

@SamixYasuke has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi maintainer, I'd like to work on this issue

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[Wilfred007]

@Wilfred007 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi can I work on this task?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Wilfred007 to this issue.

[drips-wave]

Congratulations, @Wilfred007! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @Wilfred007: 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 @Wilfred007 as part of the Stellar Wave Program's 3rd Wave 🥳

😎 @Wilfred007: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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