[#140] Write integration tests for balance caching behavior with Redis

[BigNathan1]

Backlog reference: #140 · backend

Summary

Write integration tests for balance caching behavior with Redis

Context

Endpoints that read and write Stellar account state — balances, trustlines, payments, and the transaction-history audit trail. Handling Horizon's async, eventually-consistent behavior correctly (retries, sequence numbers, mapped error codes) matters more here than almost anywhere else in the backend.

Foundation work to reach a working single-community MVP: infrastructure, schema, core CRUD APIs, the Stellar contract layer, the base frontend, wallet auth, tests, CI, and a hardened security baseline. Every later phase assumes this phase is solid.

Acceptance Criteria

• Write integration tests for balance caching behavior with Redis — implemented as described above.
• Input is validated (Zod, matching the project convention) and errors follow the existing { data, meta?, error? } response envelope.
• The change has unit and/or integration test coverage under backend/src/**/__tests__/.
• Public API surface changes are reflected in the OpenAPI spec (docs/openapi.yaml).

Definition of Done

• Implemented on a branch and opened as a PR referencing this issue (Closes #140 won't auto-link since this is a repo-internal reference — mention the issue number GitHub assigned instead).
• CI is green (lint, typecheck, tests, build).
• Reviewed against CONTRIBUTING.md conventions (Conventional Commits, code style).
• CHANGELOG.md updated under [Unreleased] if the change is user- or API-visible.
• Merged to main.

---

Generated from the CoopLumen backlog (issue.md). See CONTRIBUTING.md for the contribution workflow.