Refactor assistant configuration management and update documentation

- Removed legacy agent profile settings from the .env.example and README, streamlining the configuration process.
- Introduced a new local YAML configuration adapter for assistant settings, allowing for easier management of assistant profiles.
- Updated backend integration documentation to clarify the behavior of assistant config sourcing based on backend URL settings.
- Adjusted various service implementations to directly utilize API keys from the new configuration structure.
- Enhanced test coverage for the new local YAML adapter and its integration with backend services.

engine/services/dashscope_tts.py

@@ -89,7 +89,7 @@ class DashScopeTTSService(BaseTTSService):
         speed: float = 1.0,
     ):
         super().__init__(voice=voice, sample_rate=sample_rate, speed=speed)
-        self.api_key = api_key or os.getenv("DASHSCOPE_API_KEY") or os.getenv("TTS_API_KEY")
+        self.api_key = api_key
         self.api_url = (
             api_url
             or os.getenv("DASHSCOPE_TTS_API_URL")