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/tests/common/test_codex.rs

@@ -732,10 +732,7 @@ impl TestCodexBuilder {
             .or_else(|| codex_utils_cargo_bin::cargo_bin("codex-code-mode-host").ok());
         let thread_manager = Arc::new_cyclic(|manager| {
             let mut extensions = self.extensions.to_builder();
-            let guardian = Arc::new(codex_guardian_v2::GuardianReviewerExtension::new(
-                manager.clone(),
-            ));
-            extensions.thread_lifecycle_contributor(guardian);
+            codex_guardian_v2::install_reviewer(&mut extensions, manager.clone());
             let thread_manager = ThreadManager::new(
                 &config,
                 auth_manager.clone(),