Add fixture-based tests for sell and buy event payloads

[Chucks1093]

Summary
Introduce reusable fixture-driven tests for contract events so payload assertions stay consistent and easy to extend.

Scope

• add fixture helpers for event assertion setup
• add payload checks for buy and sell related events
• keep fixtures minimal and reusable across event tests

Acceptance Criteria

• event assertions use fixtures instead of repeated ad hoc setup
• buy and sell payload fields are explicitly validated
• tests are stable and readable for new contributors

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

[Obiajulu-gif]

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

My Chief, I will love to work on this issue

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

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