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/core/src/mcp_tool_call.rs

@@ -2136,7 +2136,6 @@ fn project_mcp_tool_approval_config_folder(
     config
         .config_layer_stack
         .layers_high_to_low()
-        .into_iter()
         .find_map(|layer| {
             if !matches!(layer.name, ConfigLayerSource::Project { .. }) {
                 return None;