Add invalid balance lookup test coverage

[Chucks1093]

Summary

Add tests for invalid balance lookups so read-method edge cases are covered clearly.

Scope

• add tests for invalid creator or holder balance lookup input
• keep the tests focused on read-method behavior
• verify predictable return or error behavior

Acceptance Criteria

• invalid balance lookup paths are covered by tests
• expected behavior is asserted clearly
• tests stay small and maintainable

Contributor note: comment on the issue first and reach out in Telegram for updates or coordination: https://t.me/accesslayerorg

[haanarrh]

@haanarrh has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would like to work on this issue, kindly assign it to me

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

[drips-wave]

Congratulations, @haanarrh! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @haanarrh: 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.

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