Honor configured proxy routes for auth refreshes (#34655)

## Why

ChatGPT token refresh requests need to follow the same configured routing policy as other authentication traffic, including environments that use the system proxy.

## What changed

- Require an `AuthRouteConfig` throughout login, logout, token refresh, personal access token, and agent identity flows.
- Build auth HTTP clients directly from that configuration instead of falling back to a default HTTP client when routing configuration is absent.

## Testing

- Add an integration test that refreshes a token through a cached system-proxy route and verifies the proxy receives the expected request.

GitOrigin-RevId: 7d54ab3219939a49921c51faf08cd4c2eabca51c

codex-rs/http-client/src/lib.rs

@@ -36,6 +36,8 @@ pub use crate::outbound_proxy::HttpClientFactory;
 pub use crate::outbound_proxy::OutboundProxyPolicy;
 pub use crate::outbound_proxy::OutboundProxyRoute;
 pub use crate::outbound_proxy::RouteFailureClass;
+#[doc(hidden)]
+pub use crate::outbound_proxy::cache_system_proxy_route_for_test;
 pub use crate::request::EncodedJsonBody;
 pub use crate::request::PreparedRequestBody;
 pub use crate::request::Request;