feat: upgrade TTS to gpt-4o-mini-tts with voice instructions

- Switch from tts-1 to gpt-4o-mini-tts for lower latency and better quality
- Change voice from alloy to coral
- Add Chinese speech instructions for natural tone control

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/services/voice-agent/src/agent.py

@@ -168,6 +168,7 @@ async def entrypoint(ctx: JobContext) -> None:
         tts = openai_plugin.TTS(
             model=settings.openai_tts_model,
             voice=settings.openai_tts_voice,
+            instructions="用自然、友好的中文语气说话，语速适中，像真人助手一样。",
             client=_oai_client_tts,
         )
     else:

packages/services/voice-agent/src/config.py

@@ -27,8 +27,8 @@ class Settings(BaseSettings):
     openai_api_key: str = ""
     openai_base_url: str = ""
     openai_stt_model: str = "gpt-4o-transcribe"
-    openai_tts_model: str = "tts-1"
-    openai_tts_voice: str = "alloy"
+    openai_tts_model: str = "gpt-4o-mini-tts"
+    openai_tts_voice: str = "coral"
 
     # Device
     device: str = "cpu"  # "cpu" or "cuda"