Enhance credential management and testing functionality

- Introduce new fields for voice, speed, and language in the AssistantConfig and ProviderCredential models to support TTS and ASR configurations.
- Update the database schema and seeding script to accommodate the new fields, ensuring backward compatibility.
- Implement credential testing endpoints and logic to validate OpenAI-compatible credentials, enhancing user experience and reliability.
- Modify frontend components to include new fields in the credential forms and improve connection testing feedback.
- Refactor related services and API interactions to support the new credential testing feature.

backend/models.py

@@ -27,6 +27,8 @@ class AssistantConfig(BaseModel):
     model: str = ""  # LLM
     asr: str = ""    # STT
     voice: str = ""  # TTS 音色
+    stt_language: str = ""
+    tts_speed: float = 1.0
     realtimeModel: str = ""
 
     enableInterrupt: bool = True