Store security risk scores as a snapshot (#38567)

## What changed

- Replace the single `category` and `score` fields in `SecurityRiskScore` with a
  deterministic map of category names to scores.
- Validate the full Guardian V2 classifier response before storing it as one
  thread extension value and, for non-ephemeral threads, one rollout item.
- Cover serialization and rollout loading with snapshots containing multiple
  score categories.

GitOrigin-RevId: 24525de691f96bd34ccfe04acab2f01aca309837

codex-rs/protocol/src/security_risk.rs

@@ -1,13 +1,13 @@
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
+use std::collections::BTreeMap;
 
-/// A durable, thread-owned security risk classifier score.
+/// A durable, thread-owned snapshot of security risk classifier scores.
 ///
 /// 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,
+    pub scores: BTreeMap<String, f64>,
 }