Implement initial greeting emission in DuplexPipeline after session activation, ensuring proper event ordering for frontend notifications.

engine/core/session.py

@@ -405,6 +405,9 @@ class Session:
                 )
             )
 
+        # Emit opener only after frontend has received session.started/config events.
+        await self.pipeline.emit_initial_greeting()
+
     async def _handle_session_stop(self, reason: Optional[str]) -> None:
         """Handle session stop."""
         if self.ws_state == WsSessionState.STOPPED: