Validate step settings against proposed permissions (#40647)

## Why

Model requirements must be evaluated against the permissions that will apply
after a settings update. Validating against the current profile can reject a
valid model-and-permission update or allow a later permission-only update to
invalidate the selected model.

## What changed

- Group model, reasoning, service-tier, personality, and approval inputs into
  `StepSettings`, with a single apply-and-validate path.
- Apply permission and environment changes before validating step settings, so
  auto-review requirements use the proposed effective permissions.
- Revalidate existing step settings when permission or environment changes alter
  their constraints, and commit the update only when the complete candidate is
  valid.

## Testing

Added unit and integration coverage for atomic model/permission updates,
permission-only revalidation, environment-derived constraints, and both
standalone and turn-start settings operations.

GitOrigin-RevId: f50dfbd300484a09acc51ee4607278f300ca1cd9

codex-rs/core/src/session/turn_input.rs

@@ -85,7 +85,7 @@ impl PreparedTurnInputSettings {
     fn would_enter_plan_mode(&self) -> bool {
         self.thread_settings_update
             .as_ref()
-            .and_then(|updates| updates.collaboration_mode.as_ref())
+            .and_then(|updates| updates.step_settings.collaboration_mode.as_ref())
             .is_some_and(|collaboration_mode| collaboration_mode.mode == ModeKind::Plan)
     }