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 {