Route backend requests through the HTTP client factory (#34490)

## Why

Backend-backed features need to use the same outbound routing configuration as the rest of Codex.

## What changed

- Build the backend client on a route-aware client pool supplied by `HttpClientFactory`.
- Pass the configured factory through app-server account requests, cloud config loading, cloud tasks, and memory rate-limit checks.
- Preserve task query encoding, authentication headers, JSON request bodies, and ChatGPT cookie handling during the migration.

## Testing

- Add backend-client tests for proxy-policy propagation, task query encoding, bearer authentication, and JSON request bodies.

GitOrigin-RevId: c17d162e0dce9cec758df7cdeecf982207c6512b

codex-rs/login/src/outbound_proxy.rs

@@ -17,7 +17,8 @@ impl AuthRouteConfig {
         }
     }
 
-    pub(crate) fn http_client_factory(&self) -> &HttpClientFactory {
+    /// Returns the HTTP client factory represented by this routing configuration.
+    pub fn http_client_factory(&self) -> &HttpClientFactory {
         &self.http_client_factory
     }
 }