Keep Guardian v2 risk scores in memory (#39304)

## What changed

- Stop writing Guardian v2 security risk scores to rollout history.
- Treat resumed and forked threads as having no previous Guardian score, so their
  first tool approval is classified and reviewed normally.
- Remove the now-unused `codex-history` dependency from the Guardian v2
  extension.

## Testing

- Update app-server coverage for new, resumed, and forked threads to wait for
  classification completion and verify the expected approval reviews.

GitOrigin-RevId: 74d9caa903a1ff9d38e577b4697b5f8970f11ebb

codex-rs/protocol/src/security_risk.rs

@@ -5,10 +5,10 @@ use serde::Deserialize;
 use serde::Serialize;
 use std::collections::BTreeMap;
 
-/// A durable, thread-owned snapshot of security risk classifier scores.
+/// A thread-owned, in-memory 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.
+/// Scores must not enter model-visible conversation context or user-visible thread
+/// item projections.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize, JsonSchema)]
 pub struct SecurityRiskScore {
     pub scores: BTreeMap<String, f64>,