Refactor backend integration and service architecture

- Removed the backend client compatibility wrapper and associated methods to streamline backend integration.
- Updated session management to utilize control plane gateways and runtime configuration providers.
- Adjusted TTS service implementations to remove the EdgeTTS service and simplify service dependencies.
- Enhanced documentation to reflect changes in backend integration and service architecture.
- Updated configuration files to remove deprecated TTS provider options and clarify available settings.

engine/tests/test_ws_protocol_session_start.py

@@ -139,7 +139,7 @@ async def test_load_server_runtime_metadata_returns_not_found_error():
             _ = assistant_id
             return {"__error_code": "assistant.not_found"}
 
-    session._backend_gateway = _Gateway()
+    session._runtime_config_provider = _Gateway()
     runtime, error = await session._load_server_runtime_metadata("assistant_demo")
     assert runtime == {}
     assert error is not None
@@ -155,7 +155,7 @@ async def test_load_server_runtime_metadata_returns_config_unavailable_error():
             _ = assistant_id
             return None
 
-    session._backend_gateway = _Gateway()
+    session._runtime_config_provider = _Gateway()
     runtime, error = await session._load_server_runtime_metadata("assistant_demo")
     assert runtime == {}
     assert error is not None