chore(core) Add approvals reviewer to UserTurn (#15426)

## Summary
Adds support for approvals_reviewer to `Op::UserTurn` so we can migrate
`[CodexMessageProcessor::turn_start]` to use Op::UserTurn

## Testing
- [x] Adds quick test for the new field

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/suite/client.rs

@@ -1310,6 +1310,7 @@ async fn user_turn_collaboration_mode_overrides_model_and_effort() -> anyhow::Re
             }],
             cwd: config.cwd.clone(),
             approval_policy: config.permissions.approval_policy.value(),
+            approvals_reviewer: None,
             sandbox_policy: config.permissions.sandbox_policy.get().clone(),
             model: session_configured.model.clone(),
             effort: Some(ReasoningEffort::Low),
@@ -1427,6 +1428,7 @@ async fn user_turn_explicit_reasoning_summary_overrides_model_catalog_default()
             }],
             cwd: config.cwd.clone(),
             approval_policy: config.permissions.approval_policy.value(),
+            approvals_reviewer: None,
             sandbox_policy: config.permissions.sandbox_policy.get().clone(),
             model: session_configured.model,
             effort: None,