Testing a Go exchange API client: mocks, httptest and testcontainers

The Go service runs locally, tests pass. Then you remember the tests are hitting the real Binance API. The next day, CI fails: rate limit exceeded, unstable network, or the exchange quietly renamed a field in its response. Tests that depend on external APIs are flaky at best, unusable in CI at worst. The problem is structural: exchanges don't have usable sandbox environments (Binance testnet exists, but with constraints that make automated integration testing difficult), rate limits are aggressi