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/deny.toml

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