Add shared constant for fee bound defaults

[Chucks1093]

Summary

Add a small shared constant source for fee bound defaults.

Scope

• define fee bound defaults in one place
• keep constants reusable across validation paths
• avoid mixing logic into the constants module

Acceptance Criteria

• fee bound defaults can be imported from one shared source
• constants are easy to adjust centrally
• module stays focused and lightweight

Contributor note: comment on the issue first and reach out in Telegram for updates or coordination: https://t.me/accesslayerorg

[Ayo-Skiller]

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

Hi, I'm a fullstack smartcontract developer and i would like to work on this project

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

[drips-wave]

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

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