Retain user instructions in Guardian context (#42844)

## Why

Guardian reviews need the original user instructions even when compaction or
transcript selection removes them from the active conversation window.

## What changed

- Capture complete user messages in host-owned retained context and include them
  as ordered authorization evidence alongside verified answers.
- Omit oversized instructions atomically and emit an explicit incomplete-evidence
  notice instead of exposing a potentially misleading partial permission.
- Mark user messages inherited by forked agents so they remain model-visible
  without becoming child-local authorization.

## Testing

- Cover retained instructions across compaction, resume, rollback, and agent
  forks, including oversized evidence and legacy context behavior.

GitOrigin-RevId: 97122509d818c90d2f87c6a1b29c99c05ad0316e

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

@@ -8783,7 +8783,10 @@ where
         session_configuration.session_source.clone(),
     );
 
-    let state = SessionState::new(session_configuration.clone());
+    let mut state = SessionState::new(session_configuration.clone());
+    if config.features.enabled(Feature::GuardianThreadContext) {
+        state.history.enable_user_message_retention();
+    }
     let (environment_manager, resolved_turn_environments) =
         resolved_environments_for_configuration(&session_configuration, &default_environments)
             .await;