Add realtime v2 event parser behind feature flag (#14537)

- Add a feature-flagged realtime v2 parser on the existing
websocket/session pipeline.
- Wire parser selection from core feature flags and map the codex
handoff tool-call path into existing handoff events.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/features.rs

@@ -180,6 +180,8 @@ pub enum Feature {
     VoiceTranscription,
     /// Enable experimental realtime voice conversation mode in the TUI.
     RealtimeConversation,
+    /// Route realtime conversations through the v2 event parser.
+    RealtimeConversationV2,
     /// Prevent idle system sleep while a turn is actively running.
     PreventIdleSleep,
     /// Use the Responses API WebSocket transport for OpenAI by default.
@@ -823,6 +825,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::RealtimeConversationV2,
+        key: "realtime_conversation_v2",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::PreventIdleSleep,
         key: "prevent_idle_sleep",