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/core/src/agent/control_tests.rs

@@ -250,6 +250,7 @@ async fn persisted_originator(thread: &CodexThread) -> String {
             | RolloutItem::EventMsg(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::WorldState(_)
+            | RolloutItem::SecurityRiskScore(_)
             | RolloutItem::TurnContext(_) => None,
         })
         .expect("session metadata should be persisted")