Implements the top-level httpx.Client that composes the full chain:
Logging → User Middlewares → Retry → Circuit Breaker → Balancer → Transport
- Response wrapper with JSON/XML/Bytes decoding and body caching
- NewJSONRequest helper with Content-Type and GetBody support
- Functional options: WithBaseURL, WithTimeout, WithRetry, WithEndpoints, etc.
- Integration tests covering retry, balancing, error mapping, and JSON round-trips
