[EmmyLee07]

Summary

Adds route-level coverage proving a remittance request can be retried with the same idempotency key after a downstream failure, without replaying or caching the 5xx response.

Linked issue

Closes #2101

Changes

• Adds a deterministic downstream service rejection to the existing remittance idempotency suite.
• Asserts the public 500 error contract, then verifies a same-key retry succeeds as a fresh 201 response.

Screenshots / Demo

Not applicable for this backend-only test change. Verification output:

Test Suites: 2 passed, 2 total
Tests:       16 passed, 16 total

Implementation notes

Uses the existing Jest/Supertest route harness and mocked remittance service/cache. No production code changes are needed: the test locks in the existing contract that 5xx responses are not cached and the in-flight reservation is released.

Testing

• cd backend && npm test -- --runInBand src/__tests__/remittanceIdempotency.test.ts src/__tests__/remittanceRoutes.test.ts - passed (16 tests).
• cd backend && npm run typecheck - passed.
• cd backend && npm run build - passed.
• cd backend && npm run lint - passed with 43 existing warnings; no warnings in the edited test.

Acceptance criteria

• Add deterministic tests for success, failure, boundary, and authorization paths described above, together with the existing route suite.
• Assert public error behavior and same-key retry after downstream failure.
• CI run and confirmation.

Checklist

• Follows CONTRIBUTING guidelines and code style.
• Lint, build, typecheck, and affected tests pass locally.
• No unrelated changes.
• Docs/changelog not required for test-only coverage.
• Rebased on latest upstream main, no conflicts.
• No AI-disclosure requirement found in repository guidance.

Notes for reviewers

CI checks will run on the pull request.