Add helper for building expected event payload struct in test assertions

[Chucks1093]

Summary
Test assertions on event payloads currently construct the expected payload inline with individual field assignments. A builder helper that constructs the full expected payload struct from named parameters makes assertions more readable and easier to update when the schema changes.

Scope

• add a test helper that builds a complete expected event payload struct from named parameters
• use the helper in at least two existing event payload assertions
• keep the helper in the test module

Acceptance Criteria

• the helper produces the correct expected payload struct for given inputs
• at least two existing event tests are updated to use the helper
• the helper is easier to read than the current inline construction

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

[brite-side0]

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[AGWAM001]

@AGWAM001 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello Maintainer, I'm excited about this issue and would like to be assigned. I'll document my approach in the PR description so the team can follow my reasoning and suggest improvements early.

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[T-kesh]

@T-kesh has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would love to work on this

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