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/json_result.rs

@@ -80,6 +80,7 @@ async fn codex_returns_json_result(model: String) -> anyhow::Result<()> {
             final_output_json_schema: Some(serde_json::from_str(SCHEMA)?),
             cwd: cwd.path().to_path_buf(),
             approval_policy: AskForApproval::Never,
+            approvals_reviewer: None,
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             model,
             effort: None,