SC-17: Ensure strict validation of token contract addresses in deposit methods

[chinweobtagaz]

Context: The contract accepts any token address, which could lead to users funding tournaments with worthless spoofed tokens.
Location: contracts/game_contract/src/lib.rs
Acceptance Criteria:

• Enforce an admin-maintained whitelist of permitted Token Contract Addresses (e.g., USDC).

[doctorlight0]

@doctorlight0 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I've analyzed the issue and understand what needs to be delivered. I'll provide a clean implementation that fulfills the acceptance criteria and maintains the project's overall quality.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @doctorlight0 to this issue.

[drips-wave]

Congratulations, @doctorlight0! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @doctorlight0: 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.

Warning

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