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/deny.toml

@@ -243,7 +243,6 @@ deny = [
         "codex-api",
         "codex-app-server",
         "codex-app-server-daemon",
-        "codex-backend-client",
         "codex-cloud-tasks",
         "codex-core",
         "codex-core-plugins",