Move Guardian reviewer lifecycle into the extension (#45537)

## Why

Guardian reviews must stop when their parent shuts down or their history is reset. Shutdown must also finish reviewer cleanup before closing the parent's persistent history, including when a review is waiting to retry after a rate limit.

## What changed

- Let the Guardian extension own reviewer startup, prewarming, and shutdown through `ThreadManager`, tracking and joining outstanding work during teardown.
- Cancel reviews on history reset or parent shutdown and reject decisions returned after cancellation, including cached extension decisions.
- Keep background prewarm previews from overwriting the active turn's model metadata.

## Testing

Extend regression coverage to verify reviewer cleanup after parent shutdown, prompt shutdown during a 60-second Guardian retry with the network request denied, and preservation of active model review requirements during prewarming.

GitOrigin-RevId: 05efc369b9a1642a10365eac8a09da77f3f6e28c

codex-rs/core/src/session/turn_context.rs

@@ -1021,12 +1021,12 @@ impl Session {
                 self.features.enabled(Feature::Personality),
             )
             .await;
-        self.services
-            .thread_extension_data
-            .insert(model_info.clone());
-
         let multi_agent_version = match multi_agent_runtime {
             TurnMultiAgentRuntime::ResolveAndStore => {
+                // A background preview must not overwrite a newer turn's model metadata.
+                self.services
+                    .thread_extension_data
+                    .insert(model_info.clone());
                 self.resolve_multi_agent_version_for_model(&model_info, &per_turn_config)
             }
             TurnMultiAgentRuntime::Preview => per_turn_config.multi_agent_version_for_model(