fix: VAD agent requires reset

Otherwise, it won't start up correctly.

ref: N25B-266

src/control_backend/main.py

@@ -117,17 +117,23 @@ async def lifespan(app: FastAPI):
         ),
     }
 
+    vad_agent_instance = None
+
     for name, (agent_class, kwargs) in agents_to_start.items():
         try:
             logger.debug("Starting agent: %s", name)
             agent_instance = agent_class(**{k: v for k, v in kwargs.items() if k != "name"})
             await agent_instance.start()
+            if isinstance(agent_instance, VADAgent):
+                vad_agent_instance = agent_instance
             logger.info("Agent '%s' started successfully.", name)
         except Exception as e:
             logger.error("Failed to start agent '%s': %s", name, e, exc_info=True)
             # Consider if the application should continue if an agent fails to start.
             raise
 
+    await vad_agent_instance.streaming_behaviour.reset()
+
     logger.info("Application startup complete.")
 
     yield