Add tests for fee split sum invariants on buy path

[Chucks1093]

Summary
Add tests for fee split sum invariants on buy path

Scope

• add tests asserting fee split sums are conserved on buy flow\n- include boundary and nominal cases\n- ensure deterministic assertions

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

[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

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