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/protocol/src/security_risk.rs

@@ -0,0 +1,13 @@
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+
+/// A durable, thread-owned security risk classifier score.
+///
+/// These records belong to rollout history only and must not enter model-visible
+/// conversation context or user-visible thread item projections.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize, JsonSchema)]
+pub struct SecurityRiskScore {
+    pub category: String,
+    pub score: f64,
+}