agent-proxy-rust-core — middleware trait, axum server engine, and upstream forwarding.

This crate provides the foundation for the agent-proxy-rust project. It defines the [ProxyMiddleware] trait (the central extension point), core domain types, error handling, configuration, an authentication layer, and the axum-based HTTP proxy engine.

Architecture

Client → Auth Layer → Router → handle_proxy_request
                                    ├── on_request chain (registration order)
                                    ├── forward to upstream
                                    └── on_response chain (reverse order)

Middleware crates (compress, bridge, model-router, cost) implement [ProxyMiddleware] and are composed via the builder.