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/backends/chat.py

@@ -0,0 +1,72 @@
+"""Backend-neutral chat contract."""
+
+from collections.abc import AsyncIterator
+from dataclasses import dataclass, field
+from typing import Any, Protocol
+
+
+@dataclass(frozen=True)
+class ChatInput:
+    """Input shared by all chat backend implementations."""
+
+    session_id: str
+    text: str
+    need_form_update: bool = False
+
+
+@dataclass(frozen=True)
+class TextDelta:
+    """A piece of raw model text.
+
+    The text may contain a partial ``<state>...</state>`` prefix. Parsing that
+    public protocol remains the responsibility of the FastAPI layer.
+    """
+
+    text: str
+
+
+@dataclass(frozen=True)
+class FormUpdate:
+    """A structured form update produced alongside model text."""
+
+    data: Any
+
+
+ChatStreamEvent = TextDelta | FormUpdate
+
+
+@dataclass(frozen=True)
+class ChatResult:
+    """Backend-neutral result for a non-streaming chat request."""
+
+    content: str
+    status_code: str | None = None
+    form_update: Any = field(default_factory=dict)
+
+
+class ChatBackend(Protocol):
+    """Contract implemented by FastGPT today and LangGraph later."""
+
+    def stream(self, chat_input: ChatInput) -> AsyncIterator[ChatStreamEvent]:
+        """Stream raw text and structured side-channel events."""
+        ...
+
+    async def complete(self, chat_input: ChatInput) -> ChatResult:
+        """Return one complete backend-neutral chat result."""
+        ...
+
+
+class ChatBackendError(Exception):
+    """Base error raised by a chat backend adapter."""
+
+
+class ChatBackendAuthenticationError(ChatBackendError):
+    """The backend rejected its configured credentials."""
+
+
+class ChatBackendRateLimitError(ChatBackendError):
+    """The backend rejected the request because of rate limiting."""
+
+
+class ChatBackendAPIError(ChatBackendError):
+    """The backend API failed or returned an invalid response."""