Enable Guardian thread context by default (#47275)

## What changed

Mark `guardianv2.thread_context` as stable and enable it by default for synchronous and asynchronous Guardian reviews. Keep it independent of the Guardian v2 `enabled` toggle and preserve explicit configuration and profile overrides. Update the configuration schema documentation to match.

## Testing

Update coverage for default configuration and sender user messages, review prompt snapshots, history retention across compaction, and checkpoint provenance. Assert that later user input invalidates an in-flight review's authorization. Keep legacy transcript budgeting coverage by explicitly disabling thread context in that test.

GitOrigin-RevId: 19afd34d7bfc0ee36bbf878089c68b732a21bc6a

codex-rs/features/src/feature_configs.rs

@@ -135,7 +135,7 @@ pub struct GuardianV2ConfigToml {
     /// Legacy setting retained for config compatibility; the backend now controls Guardian billing.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub free_guardian: Option<bool>,
-    /// Use thread-owned context for sync and async Guardian. Defaults to false.
+    /// Use thread-owned context for sync and async Guardian. Defaults to true.
     /// Independent of the Guardian v2 `enabled` toggle.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub thread_context: Option<bool>,

codex-rs/features/src/lib.rs

@@ -1627,8 +1627,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::GuardianThreadContext,
         key: "guardianv2.thread_context",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::GuardianReuseParentCompaction,

codex-rs/features/src/tests.rs

@@ -182,7 +182,8 @@ fn guardian_thread_context_resolves_nested_config_and_profile_overrides() {
     let enabled_context = "[guardianv2]\nthread_context = true";
     let disabled_context = "[guardianv2]\nthread_context = false";
     for (base, profile, enabled) in [
-        ("", "", false),
+        ("", "", true),
+        ("guardianv2 = false", "", true),
         (disabled_context, "", false),
         (enabled_context, "", true),
         (enabled_context, disabled_context, false),
@@ -207,9 +208,7 @@ fn guardian_thread_context_resolves_nested_config_and_profile_overrides() {
             FeatureOverrides::default(),
         );
         let mut expected = Features::with_defaults();
-        if enabled {
-            expected.enable(Feature::GuardianThreadContext);
-        }
+        expected.set_enabled(Feature::GuardianThreadContext, enabled);
         assert_eq!(features.enabled_features(), expected.enabled_features());
     }
 }