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/exec_policy.rs

@@ -51,6 +51,7 @@ async fn submit_user_turn(
             final_output_json_schema: None,
             cwd: test.cwd_path().to_path_buf(),
             approval_policy,
+            approvals_reviewer: None,
             sandbox_policy,
             model: session_model,
             effort: None,
@@ -131,6 +132,7 @@ async fn execpolicy_blocks_shell_invocation() -> Result<()> {
             final_output_json_schema: None,
             cwd: test.cwd_path().to_path_buf(),
             approval_policy: AskForApproval::Never,
+            approvals_reviewer: None,
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             model: session_model,
             effort: None,