test(creditline): add unit tests for repay_installment error paths

[Adeyemi-cmd]

close #1

PR Description

Summary

Adds isolated unit tests for repay_installment() in the creditline
contract. Previously the function only had incidental coverage via
end-to-end flows, leaving all error paths untested.

Changes

contracts/creditline-contract/src/tests.rs

• setup_loan_with_schedule(t, borrower, n_installments) —
  helper that registers a vendor, builds an N-installment schedule,
  and calls create_loan. Returns (loan_id, vendor).

• test_repay_installment_happy_path — pays 500 for
  installment 0, asserts paid == true, paid_at == 5000,
  remaining_balance decremented by 500, second installment
  untouched, loan still Active.

• test_repay_installment_double_pay_rejected — pays the same
  slot twice; expects InstallmentAlreadyPaid (core: add contract storage migration utilities #24).

• test_repay_installment_out_of_bounds — calls index
  installments.len() (2) on a 2-installment loan; expects
  InvalidInstallmentIndex (core: implement vendor verification and approval flow #23).

• test_repay_installment_non_borrower_rejected — calls from
  intruder address; expects UnauthorizedRepayer (core: implement interest accrual calculation on-chain #14).

• test_repay_installment_zero_amount_rejected — calls with
  amount = 0; expects InvalidRepaymentAmount (core: implement guarantee deposit locking in creditline #13).

context/progress-tracker.md

• Updated test count (93 → 98 existing + new).
• Removed the open question about missing repay_installment tests.

Testing

• cargo build — zero errors
• cargo test -p creditline-contract — 101 passed, 0 failed, 4 ignored
• No existing tests modified or weakened