Route cloud environment discovery through the HTTP client pool (#34491)

## What changed

- Build a route-aware API client from the cloud backend's configured HTTP
  factory and reuse it for environment listing and autodetection.
- Replace direct `reqwest` environment requests with a small injectable HTTP
  boundary while preserving status, content-type, body, and header handling.
- Centralize asynchronous environment-list loading around the initialized
  backend context.

## Testing

- Add coverage for production header forwarding and response decoding.
- Verify repository-specific lookup, global fallback, endpoint selection,
  deduplication, and merged environment metadata with a fake HTTP client.

GitOrigin-RevId: 2e6812015ec3eb01773184541a1fad135ed53edb

codex-rs/Cargo.lock

@@ -2436,10 +2436,10 @@ dependencies = [
  "codex-tui",
  "codex-utils-cli",
  "crossterm",
+ "http 1.4.0",
  "owo-colors",
  "pretty_assertions",
  "ratatui",
- "reqwest 0.12.28",
  "serde",
  "serde_json",
  "supports-color 3.0.2",