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/backend-client/Cargo.toml

@@ -16,7 +16,8 @@ workspace = true
 anyhow = "1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
+http = { workspace = true }
+url = { workspace = true }
 codex-backend-openapi-models = { path = "../codex-backend-openapi-models" }
 codex-api = { workspace = true }
 codex-http-client = { workspace = true }
@@ -26,3 +27,4 @@ codex-protocol = { workspace = true }
 
 [dev-dependencies]
 pretty_assertions = "1"
+tokio = { workspace = true, features = ["macros", "rt"] }