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/common/test_codex.rs

@@ -723,6 +723,7 @@ impl TestCodex {
                 final_output_json_schema: None,
                 cwd: self.config.cwd.clone(),
                 approval_policy,
+                approvals_reviewer: None,
                 sandbox_policy,
                 model: session_model,
                 effort: None,