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/docs/extension_ports.md

@@ -0,0 +1,47 @@
+# Engine Extension Ports (Draft)
+
+This document defines the draft port set used to keep core runtime extensible.
+
+## Port Modules
+
+- `core/ports/control_plane.py`
+  - `AssistantRuntimeConfigProvider`
+  - `ConversationHistoryStore`
+  - `KnowledgeRetriever`
+  - `ToolCatalog`
+  - `ControlPlaneGateway`
+- `core/ports/llm.py`
+  - `LLMServiceSpec`
+  - `LLMPort`
+  - optional extensions: `LLMCancellable`, `LLMRuntimeConfigurable`
+- `core/ports/tts.py`
+  - `TTSServiceSpec`
+  - `TTSPort`
+- `core/ports/asr.py`
+  - `ASRServiceSpec`
+  - `ASRPort`
+  - optional extensions: `ASRInterimControl`, `ASRBufferControl`
+- `core/ports/service_factory.py`
+  - `RealtimeServiceFactory`
+
+## Adapter Layer
+
+- `app/service_factory.py` provides `DefaultRealtimeServiceFactory`.
+- It maps resolved provider specs to concrete adapters.
+- Core orchestration (`core/duplex_pipeline.py`) depends on the factory port/specs, not concrete provider classes.
+
+## Provider Behavior (Current)
+
+- LLM:
+  - supported providers: `openai`, `openai_compatible`, `openai-compatible`, `siliconflow`
+  - fallback: `MockLLMService`
+- TTS:
+  - supported providers: `dashscope`, `openai_compatible`, `openai-compatible`, `siliconflow`
+  - fallback: `MockTTSService`
+- ASR:
+  - supported providers: `openai_compatible`, `openai-compatible`, `siliconflow`
+  - fallback: `BufferedASRService`
+
+## Notes
+
+- This is a draft contract set; follow-up work can add explicit capability negotiation and contract-version fields.