[CT-06] Implement Resource Credit Deduction Validator with balance guard in Rust

[devwums]

Problem

The resource_credits contract can deduct credits without validating the balance, potentially allowing negative credit balances that corrupt member state.

Proposed Solution

Create contracts/cntr/credit_deduction.rs implementing pub fn validate_deduction(current_balance: i128, deduction_amount: i128) -> Result<i128, &'static str>. Returns Ok(new_balance) only when deduction_amount > 0 and current_balance >= deduction_amount. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/credit_deduction.rs
• Returns Err("Deduction amount must be positive") when deduction_amount <= 0
• Returns Err("Insufficient credits") when balance would go negative
• Returns Ok(current_balance - deduction_amount) on success
• At least 6 unit tests covering: success, exact balance, insufficient, zero deduction, negative deduction
• Compiles with cargo test

[BashMan11]

@BashMan11 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi there I would like to take on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @BashMan11 to this issue.

[drips-wave]

Congratulations, @BashMan11! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @BashMan11: 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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[BashMan11]

Resolved in PR #1042. Implementation lives in contracts/cntr/ as specified.

[BashMan11]

Implementation complete. PR: #1042 — will close this issue on merge.