Persist security risk scores in rollout history (#38363)

## What changed

- Add a `SecurityRiskScore` rollout item containing a category and numeric score.
- Persist the item in both thread history modes while excluding it from model context, user-visible thread history, search text, forks, and reconstructed conversation history.
- Re-export the score type from the extension API.

## Testing

- Cover serialization, persistence and loading, thread history projection, session reconstruction, append planning, and memory filtering.

GitOrigin-RevId: 1926fe366aeaa75052708a6da589f45a38eefb52

codex-rs/state/src/runtime/threads.rs

@@ -1293,6 +1293,7 @@ pub(super) fn extract_memory_mode(items: &[RolloutItem]) -> Option<String> {
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::WorldState(_)
+        | RolloutItem::SecurityRiskScore(_)
         | RolloutItem::EventMsg(_) => None,
     })
 }