Clarify config layer iteration APIs (#36774)

## What changed

- Replace the ordering enum and `include_disabled` boolean with named
  `ConfigLayerStack` iterators for each precedence direction.
- Keep enabled-only iteration separate from `all_layers_*` iteration, which
  includes disabled layers.
- Update config consumers to use the iterator matching their precedence and
  disabled-layer requirements without allocating an intermediate `Vec`.

## Testing

- Add coverage that verifies ordering and disabled-layer filtering for all four
  iterators.

GitOrigin-RevId: d14df3db4a2eae80ba97cdec246bff405e6c5f3d

codex-rs/tui/src/chatwidget.rs

@@ -116,7 +116,6 @@ use codex_app_server_protocol::TurnCompletedNotification;
 use codex_app_server_protocol::TurnPlanStepStatus;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
-use codex_config::ConfigLayerStackOrdering;
 use codex_config::Constrained;
 use codex_config::ConstraintResult;
 use codex_config::types::ApprovalsReviewer;