[CT-18] Write Payment Verification Test Suite (10+ tests) in Rust

[devwums]

Problem

Payment validation logic in manage_hub has no dedicated test suite. Payment processing bugs — especially around tolerance and boundary values — can go undetected.

Proposed Solution

Create contracts/cntr/tests/payment_verification_tests.rs. Write at least 10 unit tests for payment validation: exact amount, overpayment (should pass), within tolerance (should pass), one stroop below tolerance (should fail), underpayment well below, zero payment, negative payment, zero expected amount, tolerance of zero, large amounts near i128 bounds. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/tests/payment_verification_tests.rs
• At least 10 test functions with descriptive names
• Tests import and use payment_validator.rs peer module
• Tolerance boundary conditions tested at both sides (inside and outside)
• All tests pass with cargo test

[distributed-nerd]

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

can i work on this

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[devbackend513-ux]

@devbackend513-ux has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i take this

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[drips-wave]

Congratulations, @devbackend513-ux! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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[drips-wave]

This issue has been completed by @devbackend513-ux as part of the Stellar Wave Program's 5th Wave 🥳

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