Add opt-in elastic budgeting for asynchronous Guardian requests (#61345)

## What changed

- Add `async_elastic_budgeting` and `async_elastic_sections` under `[features.guardianv2]`, independently of synchronous budgeting and disabled by default.
- Apply section limits and elastic allocation to new classifier evidence after accounting for setup and retained conversation history. Preserve the configured elastic policy when applying model defaults.
- Keep permission context whole during elastic allocation and record truncation observations when image preparation rejects an image.

## Testing

Add coverage for configuration validation, fixed and elastic conversation budgeting, retained history and cursor preservation, and complete classifier requests that retain evidence beyond the fixed entry limit.

GitOrigin-RevId: 2955f0482f7c3e18325e296fc2ccb6592ffcb73a

codex-rs/features/src/feature_configs.rs

@@ -163,6 +163,12 @@ pub struct GuardianElasticSectionConfigToml {
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, JsonSchema)]
 #[serde(deny_unknown_fields)]
 pub struct GuardianV2ConfigToml {
+    /// Enable elastic allocation for asynchronous Guardian requests. Defaults to false.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub async_elastic_budgeting: Option<bool>,
+    /// Section limits validated when asynchronous elastic budgeting is enabled.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub async_elastic_sections: Option<BTreeMap<String, GuardianElasticSectionConfigToml>>,
     /// Enable elastic allocation for synchronous Guardian requests. Defaults to false.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub sync_elastic_budgeting: Option<bool>,

codex-rs/features/src/tests.rs

@@ -332,6 +332,8 @@ max_recent_non_user_entries = 12
     assert_eq!(
         features.guardianv2,
         Some(FeatureToml::Config(crate::GuardianV2ConfigToml {
+            async_elastic_budgeting: None,
+            async_elastic_sections: None,
             sync_elastic_budgeting: None,
             sync_elastic_sections: None,
             transcript_mode: None,