Add regression test for key balance read returning zero for address with no buys

[Chucks1093]

Summary
A valid wallet address that has never bought keys for a creator should return a zero balance rather than an error or null. A regression test for this state confirms the balance read handles uninitialized holders gracefully.

Scope

• add a regression test that reads the key balance for a wallet address that has never purchased keys
• assert the returned balance is zero
• assert no error is produced for the uninitialized address

Acceptance Criteria

• a key balance read for an address with no buys returns zero
• no error is produced for the uninitialized address
• the test uses a distinct address that has not interacted with the contract

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

[AbuJulaybeeb]

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

Hi Maintainer
I would like to hop on this issue

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

[drips-wave]

Congratulations, @AbuJulaybeeb! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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