feat(infra): Phase 2 — API client abstraction, retry logic, rate limiting
Extract shared APIClient from HCloudClient/HRobotClient/CloudNSClient with
configurable retry (exponential backoff + jitter), 429 rate-limit handling
(Retry-After header), and functional options (WithHTTPClient, WithRetry,
WithAuth, WithPrefix). 30 new client_test.go tests covering retry exhaustion,
rate-limit queuing, context cancellation, and integration with all 3 providers.
DigitalOcean: no code existed, removed stale doc references. 66 infra tests
pass, race-clean.
Co-Authored-By: Virgil <virgil@lethean.io>
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