Refactor chat backend integration and introduce new backend-neutral architecture

- Replaced direct FastGPT client usage with a backend-neutral chat interface, allowing for improved flexibility and maintainability.
- Introduced a new `ChatBackend` protocol and implemented `FastGPTBackend` to handle chat operations.
- Updated the chat endpoint to utilize the new backend structure, enhancing the handling of chat requests and responses.
- Added comprehensive tests to ensure compatibility and functionality of the new backend integration.

src/core/fastgpt_client.py

@@ -1,17 +1,20 @@
-"""FastGPT client dependency injection."""
+"""FastGPT client and chat backend dependency injection."""
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastgpt_client import AsyncChatClient
+from ..backends.chat import ChatBackend
+from ..backends.fastgpt import FastGPTBackend
 from .config import Config
 
 # Global client instance
 _fastgpt_client: AsyncChatClient | None = None
+_chat_backend: ChatBackend | None = None
 
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Manage FastGPT client lifecycle."""
-    global _fastgpt_client
+    global _chat_backend, _fastgpt_client
     Config.validate()
     
     # Initialize client
@@ -24,12 +27,15 @@ async def lifespan(app: FastAPI):
         enable_logging=Config.DEBUG,
     )
     await _fastgpt_client.__aenter__()
-    
-    yield
-    
-    # Cleanup
-    if _fastgpt_client:
-        await _fastgpt_client.__aexit__(None, None, None)
+    _chat_backend = FastGPTBackend(_fastgpt_client)
+
+    try:
+        yield
+    finally:
+        _chat_backend = None
+        if _fastgpt_client:
+            await _fastgpt_client.__aexit__(None, None, None)
+        _fastgpt_client = None
 
 
 def get_fastgpt_client() -> AsyncChatClient:
@@ -37,3 +43,10 @@ def get_fastgpt_client() -> AsyncChatClient:
     if _fastgpt_client is None:
         raise RuntimeError("FastGPT client not initialized")
     return _fastgpt_client
+
+
+def get_chat_backend() -> ChatBackend:
+    """Get the backend-neutral chat service."""
+    if _chat_backend is None:
+        raise RuntimeError("Chat backend not initialized")
+    return _chat_backend