Add regression test for buy revert when creator is not registered

[Chucks1093]

Summary
The buy quote method is tested against a missing creator, but the actual buy execution path for an unregistered creator is not covered by a regression test. Confirming the buy reverts with the expected error prevents a silent failure mode.

Scope

• add a regression test that calls the buy function for a creator address that has not been registered
• assert the call reverts with the expected not-registered error variant
• assert no state changes occur as a result of the failed call

Acceptance Criteria

• a buy against an unregistered creator reverts with the correct error
• no supply, balance, or fee state is modified by the failed call
• the test is distinct from the existing buy quote missing-creator test

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

[razeprasine]

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

Hi Maitnainer, I will love to fix this issue

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

[drips-wave]

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

🧑‍💻 @razeprasine: 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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