Add event test for buy payload fee fields

[Chucks1093]

Summary
Add event test for buy payload fee fields

Scope

• add event tests validating buy event fee-related payload fields\n- verify field presence and value mapping\n- keep test fixtures reusable

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

[Jenix777]

@Jenix777 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hello Maintainer,

I hope you're doing well. I came across issue #199 and would like to request to be assigned to it.

I've reviewed the scope and understand what's needed — adding event tests that validate the fee-related payload fields in buy events, verifying both field presence and value mapping, with reusable test fixtures.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Jenix777 to this issue.

[drips-wave]

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

🧑‍💻 @aistitchpay-sys: 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 🌊