[1/5] [codex] sync managed-layer bundle schema (#31285)

## Why

The backend config-bundle contract now exposes managed configuration in
`managed_layers`, split into `baseline` and `system_overlay`. The Rust
transport models need to match that contract before any runtime behavior
changes.

## What changed

- add the generated `DeliveredManagedLayers` model
- model `baseline` and `system_overlay` as required arrays
- expose optional/null `managed_layers` on delivered config and
requirements documents
- retain `enterprise_managed` for transport compatibility

This PR changes transport types only; cloud-config runtime behavior is
unchanged.

## Stack

1 of 4. Next: #31286.

## Validation

- `just test -p codex-backend-openapi-models -p codex-backend-client`
- `just test -p codex-cloud-config`
- revalidated against the current generated backend schema

codex-rs/backend-client/src/lib.rs

@@ -15,6 +15,7 @@ pub use types::ConfigBundleResponse;
 pub use types::ConsumeRateLimitResetCreditCode;
 pub use types::ConsumeRateLimitResetCreditResponse;
 pub use types::DeliveredConfigToml;
+pub use types::DeliveredManagedLayers;
 pub use types::DeliveredRequirementsToml;
 pub use types::DeliveredTomlFragment;
 pub use types::PaginatedListTaskListItem;