Persist verified user answers in Guardian thread context (#42579)

## Why

Guardian authorization reviews need host-verified `request_user_input` answers to
remain aligned with thread history across compaction, resume, and rollback.

## What changed

- Record verified question-and-answer pairs in retained thread context when
  `GuardianThreadContext` is enabled, and source both local and delegated Guardian
  review evidence from that context.
- Preserve complete answer records within the evidence budget. Mark authorization
  context incomplete when records are unavailable or oversized, and defer cached
  fast approvals in that case.
- Keep the existing runtime-only answer path for threads that do not enable the
  thread-owned context.

## Testing

- Cover answer retention through compaction and resume, removal after rollback,
  source-call eviction, oversized answers, delegated review evidence, and legacy
  behavior.

GitOrigin-RevId: 01f6f4541515434a0d00fdde318ff59f2f8bd2eb

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

@@ -230,6 +230,7 @@ mod mcp_refresh;
 mod mcp_runtime;
 pub(crate) mod multi_agents;
 mod realtime_history;
+mod retained_context;
 mod review;
 mod rollout_budget;
 mod rollout_reconstruction;

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

@@ -0,0 +1,26 @@
+//! Records host-verified facts at the same persistence boundary as context checkpoints.
+
+use codex_history::RetainedContextEvent;
+use codex_history::RolloutItem;
+
+use super::Session;
+use super::thread_settings;
+
+impl Session {
+    pub(crate) async fn record_retained_context(&self, mut event: RetainedContextEvent) {
+        event.bound();
+        // Share the checkpoint persistence lock so a fact cannot land on the wrong side
+        // of the checkpoint/suffix boundary. Ephemeral threads use the same live state.
+        let _guard = thread_settings::acquire_persistence_lock(self).await;
+        if self
+            .state
+            .lock()
+            .await
+            .history
+            .record_retained_context(&event)
+        {
+            self.persist_rollout_items(&[RolloutItem::RetainedContext(event)])
+                .await;
+        }
+    }
+}

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

@@ -837,6 +837,10 @@ impl Session {
             thread_id.to_string(),
             thread_extension_init,
         );
+        // Select the answer path once, before extensions or tool handlers observe the thread.
+        thread_extension_data.insert(crate::context::GuardianReviewEvidence::from_features(
+            &config.features,
+        ));
         // Kick off independent async setup tasks in parallel to reduce startup latency.
         //
         // - initialize thread persistence with new or resumed session info