## Why
Cloud task requests can use saved ChatGPT credentials, so their destination must be constrained before authentication is loaded or any request is made.
## What changed
- Validate `CODEX_CLOUD_TASKS_BASE_URL` against trusted ChatGPT HTTPS origins on port 443 and reject URLs containing user information, queries, or fragments.
- Disable redirects for cloud task backend and environment-discovery clients so credentials are never forwarded to a redirect destination.
- Normalize the validated base URL before configuring authentication and HTTP clients.
## Testing
Add a CLI integration test that verifies a custom cloud backend is rejected without receiving a request, while the staging origin continues to use explicit token authentication.
GitOrigin-RevId: f0dddf363b4e44ea3530f81d9a817e978c15ead2
