Add helper for building deterministic test wallet addresses

[Chucks1093]

Summary
Contract tests that need multiple distinct wallet addresses currently construct them inline with varying approaches, making fixtures inconsistent. A small helper that generates deterministic test addresses from an index or seed keeps fixtures uniform and easy to read.

Scope

• add a test helper that returns a deterministic Soroban Address from a given index or seed string
• use the helper in at least one existing test that constructs addresses manually
• keep the helper scoped to the test module

Acceptance Criteria

• the helper returns the same address for the same input on every call
• addresses produced by different inputs are distinct
• at least one existing test is updated to use the helper

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 Maintainer, 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.

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