[1] Add dedicated tests for `repay_installment()`

[EmeditWeb]

Problem

The repay_installment() function in contracts/creditline-contract/src/lib.rs lacks isolated unit tests. The only existing coverage is incidental via end-to-end flow tests, leaving error paths (double-pay, out-of-bounds, unauthorized, zero-amount) untested. Without these, regressions in repayment logic can silently break borrower balances.

Context

Repayment correctness is the most safety-critical operation in StepFi — a bug here can either lock learners out of repaying or allow them to pay twice. Sponsors lose trust if installments are mis-accounted, and the reputation contract derives scoring from these calls. This must be airtight before mainnet.

Before Starting

Read these context files first:

• context/architecture-context.md
• context/code-standards.md
• context/progress-tracker.md
• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/tests.rs

What To Build

1. Add a helper setup_loan_with_schedule(env, borrower, amount, n_installments) at the top of tests.rs that initializes the contract, creates a loan, and approves it.
2. Test repay_installment_happy_path: pay installment 0, assert Installment.paid == true, assert outstanding balance decremented by exact amount.
3. Test repay_installment_double_pay_rejected: pay installment 0, then call again — assert ContractError::InstallmentAlreadyPaid is returned.
4. Test repay_installment_out_of_bounds: call with index installments.len() — assert ContractError::InvalidInstallmentIndex.
5. Test repay_installment_non_borrower_rejected: call from a wallet other than the loan borrower — assert auth panic via env.mock_auths() mismatch.
6. Test repay_installment_zero_amount_rejected: ensure zero-amount payments are explicitly rejected with ContractError::InvalidAmount.

Files To Touch

• contracts/creditline-contract/src/tests.rs
• contracts/creditline-contract/src/errors.rs (if InvalidInstallmentIndex / InstallmentAlreadyPaid not yet defined)

Acceptance Criteria

• 5 new #[test] functions covering each scenario above
• cargo test -p creditline-contract passes with all new tests green
• Each test uses Env::default() and mock_all_auths() where appropriate
• No existing test is modified or weakened
• Test names follow the repay_installment_<scenario> convention
• Coverage report shows repay_installment() branch coverage ≥ 90%

Mandatory Checks Before PR

• cargo build passes with zero errors
• cargo test — all 93 existing tests still pass
• require_auth() is FIRST line of every mutating function
• extend_ttl() called after EVERY persistent storage write
• New unit tests written for every new function
• context/progress-tracker.md updated

[Adeyemi-cmd]

Can I work on this pls?

[grantfox-oss]

🦊 GrantFox — @Adeyemi-cmd has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Adeyemi-cmd's PR #25 was approved and merged by @EmeditWeb.

🏆 @Adeyemi-cmd: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (48 total points). Track your full progress on GrantFox.

👏 Great work, @Adeyemi-cmd! Keep contributing to StepFi-app.