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/ext/extension-api/src/lib.rs

@@ -15,6 +15,7 @@ pub use capabilities::ResponseItemInjectionFuture;
 pub use capabilities::ResponseItemInjector;
 pub use codex_context_fragments::ContextualUserFragment;
 pub use codex_protocol::models::ResponseItem;
+pub use codex_protocol::security_risk::SecurityRiskScore;
 pub use codex_tools::ConversationHistory;
 pub use codex_tools::ExtensionTurnItem;
 pub use codex_tools::FunctionCallError;