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/cloud-config/Cargo.toml

@@ -12,6 +12,7 @@ base64 = { workspace = true }
 chrono = { workspace = true, features = ["serde"] }
 codex-backend-client = { workspace = true }
 codex-config = { workspace = true }
+codex-http-client = { workspace = true }
 codex-core = { workspace = true }
 codex-login = { workspace = true }
 codex-otel = { workspace = true }