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/sqlite_state.rs

@@ -395,6 +395,7 @@ async fn mcp_call_marks_thread_memory_mode_polluted_when_configured() -> Result<
             final_output_json_schema: None,
             cwd: test.cwd_path().to_path_buf(),
             approval_policy: AskForApproval::Never,
+            approvals_reviewer: None,
             sandbox_policy: SandboxPolicy::new_read_only_policy(),
             model: test.session_configured.model.clone(),
             effort: None,