Add helper for asserting event field count matches expected schema

[Chucks1093]

Summary
Event tests assert individual field values but not the total number of fields in the payload. A helper that asserts the event contains exactly the expected number of fields catches accidental additions or removals that would break indexers.

Scope

• add a test helper that asserts an emitted event contains exactly the expected number of fields
• apply the helper to at least two existing event tests
• document the expected field count alongside each event schema

Acceptance Criteria

• the helper fails if the event contains more or fewer fields than expected
• at least two existing event tests are updated to use the helper
• the expected field counts are defined as named constants

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

[Blckjiggy]

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

Can this be assigned to me? I'd like to work on this

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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 want to work on this

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[drips-wave]

Congratulations, @T-kesh! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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