[CT-21] Write Resource Credits Test Suite (10+ tests) in Rust

[devwums]

Problem

The resource_credits contract has no dedicated test suite. Deduction and top-up edge cases — especially zero-balance deduction and maximum-balance top-up — are untested and commonly cause off-by-one errors.

Proposed Solution

Create contracts/cntr/tests/resource_credits_tests.rs. Write at least 10 unit tests covering: deduct from full balance (success), deduct exact balance leaving zero (success), deduct more than balance (fail), deduct zero (fail), deduct negative (fail), top-up from zero, top-up to exact max balance, top-up one above max (fail), zero top-up (fail), large balance accumulation. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/tests/resource_credits_tests.rs
• At least 10 test functions
• Tests import credit_deduction.rs and credit_topup.rs peer modules
• Deduction-to-zero edge case is explicitly and separately tested
• All tests pass with cargo test

[Ahmedscreativeverse]

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

i can do this considering my years of experience

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

[Mkalbani]

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

Hi Maintainers, can I work on this?

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

[drips-wave]

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

🧑‍💻 @Mkalbani: 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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