Consolidate Guardian reviewer lifecycle ownership (#45672)

## What changed

- Use cancellation guards to tie reusable reviewers and temporary forks to their lifetimes, including when a review future is dropped. Replace cancelled reusable reviewers before reuse.
- Share `ReviewerTasks` between the pool and `ThreadManager`, and wait for tracked cleanup during pool shutdown. Remove the separate session shutdown protocol.
- Move denial cleanup from core task handling into Guardian's turn start, stop, and abort hooks. Expose `install_reviewer` to register both thread and turn lifecycle contributors together.

## Testing

Add an integration test that exercises stale denial cleanup at turn start, completion, and interruption, including starting another turn after interruption.

GitOrigin-RevId: 85badb235309c1e1576547d1746f7ee9fc66d545

codex-rs/core/src/session/tests/guardian_tests.rs

@@ -1330,48 +1330,3 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
     );
     drop(io);
 }
-
-#[test_case(TerminalEventKind::TurnComplete; "completion")]
-#[test_case(TerminalEventKind::TurnAborted; "interruption")]
-#[tokio::test]
-async fn terminal_turn_clears_extension_owned_denials(terminal: TerminalEventKind) {
-    let (session, turn, events) = make_session_and_context_with_rx().await;
-    let finish = Arc::new(tokio::sync::Notify::new());
-    session
-        .spawn_task(
-            Arc::clone(&turn),
-            Vec::new(),
-            HeldStepTask {
-                kind: TaskKind::Regular,
-                finish: Arc::clone(&finish),
-            },
-        )
-        .await;
-    let denials =
-        codex_guardian_reviewer::ReviewDenials::for_thread(&session.services.thread_extension_data);
-    for _ in 0..2 {
-        assert_eq!(
-            denials.record_denial(&turn.sub_id, turn.model_info()).await,
-            None
-        );
-    }
-    match terminal {
-        TerminalEventKind::TurnComplete => finish.notify_one(),
-        TerminalEventKind::TurnAborted => {
-            session.abort_all_tasks(TurnAbortReason::Interrupted).await
-        }
-    }
-    recv_terminal_event(&events, terminal).await;
-    // Delivery precedes accounting cleanup. Wait for the runtime to finish the turn.
-    timeout(Duration::from_secs(/*secs*/ 5), async {
-        while session.active_turn.lock().await.is_some() {
-            tokio::task::yield_now().await;
-        }
-    })
-    .await
-    .expect("turn becomes idle");
-    assert_eq!(
-        denials.record_denial(&turn.sub_id, turn.model_info()).await,
-        None
-    );
-}