chore: clean imports and restore test coverage

src/voice_assistant/audio_gateway/app/main.py

@@ -36,7 +36,6 @@ import json
 import logging
 from fastapi import FastAPI, WebSocket, WebSocketDisconnect
 from pydantic import BaseModel, ValidationError
-import os
 
 import asyncio
 
@@ -45,7 +44,6 @@ from voice_assistant.stt.adapter import STTAdapter
 from voice_assistant.tts.adapter import TTSAdapter
 from voice_assistant.turn_manager import (
     TurnManager,
-    TurnState,
 )
 from voice_assistant.agent.adapter import AgentAdapter
 
@@ -72,7 +70,6 @@ from voice_assistant.audio_gateway.app.protocol import (
     AUDIO_SAMPLE_RATE,
     AUDIO_CHANNELS,
     AudioChunk,
-    AudioStart,
     AudioStop,
     Interrupt,
     SessionEnd,