Mark multi-agent v2 as stable (#34383)

## What changed

- Classify `multi_agent_v2` as stable while keeping it disabled by default.
- Exclude structured `multi_agent_v2` configuration from under-development feature warnings.

## Testing

- Update the warning-event test to verify that only the remaining under-development feature is reported.

GitOrigin-RevId: 12cb661dde9bc7b5aeae2f83476cb5ff75e0c959

codex-rs/features/src/lib.rs

@@ -1049,7 +1049,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::MultiAgentV2,
         key: "multi_agent_v2",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Stable,
         default_enabled: false,
     },
     FeatureSpec {

codex-rs/features/src/tests.rs

@@ -531,7 +531,7 @@ fn unstable_warning_event_only_mentions_enabled_under_development_features() {
 }
 
 #[test]
-fn unstable_warning_event_mentions_enabled_structured_under_development_feature() {
+fn unstable_warning_event_ignores_enabled_structured_stable_feature() {
     let configured_features: Table = toml::from_str(
         r#"
 multi_agent_v2 = { enabled = true, tool_namespace = "agents" }
@@ -556,7 +556,7 @@ code_mode = true
         panic!("expected warning event");
     };
     assert_eq!(
-        "Under-development features enabled: code_mode, multi_agent_v2. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in /tmp/config.toml.".to_string(),
+        "Under-development features enabled: code_mode. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in /tmp/config.toml.".to_string(),
         message
     );
 }