[CT-14] Implement Payment Amount Validator with tolerance support in Rust

[devwums]

Problem

The payment validation logic in the manage_hub contract accepts any payment amount. There is no check against the expected subscription price, allowing underpayment to silently succeed.

Proposed Solution

Create contracts/cntr/payment_validator.rs implementing pub fn validate_payment_amount(received_stroops: i128, expected_stroops: i128, tolerance_stroops: i128) -> Result<(), &'static str>. The payment is valid when received_stroops >= expected_stroops - tolerance_stroops. Also validates that expected_stroops > 0. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/payment_validator.rs
• Returns Ok(()) when received is within tolerance of expected
• Returns Err("Payment amount insufficient") when received is below the tolerance threshold
• Returns Err("Expected amount must be positive") when expected_stroops <= 0
• At least 6 unit tests: exact amount, within tolerance, one stroop below tolerance, overpayment, zero expected
• Compiles with cargo test

[authenticeasy-sys]

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

can take this

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

[drips-wave]

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

🧑‍💻 @authenticeasy-sys: 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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