Keep the latest Guardian risk score during concurrent sampling (#38580)

## Why

Concurrent Guardian samples can finish out of order, allowing an older sample to
replace the thread's newer in-memory security risk score.

## What changed

- Record when each `SecurityRiskScore` sample starts while preserving compatibility
  with records that have no timestamp.
- Add `ExtensionData::insert_if` so checking and replacing a typed attachment happen
  under the same lock.
- Update Guardian V2 to replace the thread's risk score only when the completed sample
  is newer than the stored one.

## Testing

- Verify concurrent conditional inserts retain the newest value.
- Verify Guardian-generated risk scores include a sampling timestamp.

GitOrigin-RevId: 6543a4e5497da4c623331050789e91613f931c4b

codex-rs/ext/guardian-v2/src/extension.rs

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 use std::sync::Weak;
+use std::time::SystemTime;
 
 use codex_core::ThreadManager;
 use codex_core::config::Config;
@@ -133,6 +134,7 @@ impl ToolLifecycleContributor for GuardianV2Extension {
         let Some(sampler) = input.thread_store.get::<LunaSampler>() else {
             return Box::pin(std::future::ready(()));
         };
+        let sampled_at = SystemTime::now();
         let event_sink = Arc::clone(&self.event_sink);
         let thread_manager = self.thread_manager.clone();
         let thread_id = input.thread_store.level_id().to_owned();
@@ -239,8 +241,15 @@ impl ToolLifecycleContributor for GuardianV2Extension {
                             .ok_or_else(|| format!("invalid security risk score for {category}"))
                     })
                     .collect::<Result<_, _>>()?;
-                let score = SecurityRiskScore { scores };
-                thread.thread_extension_data().insert(score.clone());
+                let score = SecurityRiskScore {
+                    scores,
+                    sampled_at: Some(sampled_at.into()),
+                };
+                thread
+                    .thread_extension_data()
+                    .insert_if(score.clone(), |previous| {
+                        previous.is_none_or(|previous| previous.sampled_at < score.sampled_at)
+                    });
                 if !ephemeral {
                     thread
                         .append_rollout_items(&[RolloutItem::SecurityRiskScore(score)])

codex-rs/ext/guardian-v2/src/extension_tests.rs

@@ -284,8 +284,10 @@ async fn contributor_samples_tool_calls_with_the_existing_luna_pool() -> Result<
         score.as_ref(),
         &SecurityRiskScore {
             scores: BTreeMap::from([("action_risk".to_string(), 0.8)]),
+            sampled_at: score.sampled_at,
         }
     );
+    assert!(score.sampled_at.is_some());
     assert_eq!(
         registry.approval_requirement(thread_store),
         ApprovalRequirement::RequireAutomaticReview
@@ -307,6 +309,7 @@ async fn contributor_samples_tool_calls_with_the_existing_luna_pool() -> Result<
 
     thread_store.insert(SecurityRiskScore {
         scores: BTreeMap::from([("action_risk".to_string(), 0.25)]),
+        sampled_at: None,
     });
     assert_eq!(
         registry.approval_requirement(thread_store),
@@ -316,6 +319,7 @@ async fn contributor_samples_tool_calls_with_the_existing_luna_pool() -> Result<
     let disabled_thread_store = ExtensionData::new("disabled-thread");
     disabled_thread_store.insert(SecurityRiskScore {
         scores: BTreeMap::from([("action_risk".to_string(), 0.8)]),
+        sampled_at: None,
     });
     assert_eq!(
         registry.approval_requirement(&disabled_thread_store),