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/rollout/src/model_context.rs

@@ -153,6 +153,7 @@ impl ModelContextScan {
             RolloutItem::EventMsg(_)
             | RolloutItem::SessionMeta(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
+            | RolloutItem::SecurityRiskScore(_)
             | RolloutItem::WorldState(_) => {}
         }