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.

test/backends/test_fastgpt_backend.py

@@ -0,0 +1,123 @@
+from types import SimpleNamespace
+
+import pytest
+
+from src.backends.chat import ChatInput, FormUpdate, TextDelta
+from src.backends.fastgpt import FastGPTBackend
+
+
+class FakeResponse:
+    def __init__(self, data):
+        self._data = data
+        self.raise_for_status_called = False
+
+    def raise_for_status(self):
+        self.raise_for_status_called = True
+
+    def json(self):
+        return self._data
+
+
+class FakeClient:
+    def __init__(self, response):
+        self.response = response
+        self.calls = []
+
+    async def create_chat_completion(self, **kwargs):
+        self.calls.append(kwargs)
+        return self.response
+
+
+@pytest.mark.asyncio
+async def test_complete_translates_fastgpt_response_to_neutral_result():
+    response = FakeResponse(
+        {
+            "choices": [
+                {
+                    "message": {
+                        "content": "<state>1002</state>请描述事故经过。"
+                    }
+                }
+            ],
+            "newVariables": {"status_code": "1002"},
+            "responseData": [
+                {
+                    "moduleName": "文本内容提取事故信息",
+                    "extractResult": {
+                        "formUpdate": '{"jdcsl": 2, "ywrysw": false}'
+                    },
+                }
+            ],
+        }
+    )
+    client = FakeClient(response)
+    backend = FastGPTBackend(client)
+
+    result = await backend.complete(
+        ChatInput(
+            session_id="session-001",
+            text="发生了交通事故",
+            need_form_update=True,
+        )
+    )
+
+    assert response.raise_for_status_called is True
+    assert result.content == "<state>1002</state>请描述事故经过。"
+    assert result.status_code == "1002"
+    assert result.form_update == {"jdcsl": 2, "ywrysw": False}
+    assert client.calls == [
+        {
+            "messages": [{"role": "user", "content": "发生了交通事故"}],
+            "chatId": "session-001",
+            "stream": False,
+            "detail": True,
+            "variables": {"needFormUpdate": True},
+        }
+    ]
+
+
+@pytest.mark.asyncio
+async def test_stream_translates_fastgpt_events_to_neutral_events(monkeypatch):
+    response = object()
+    client = FakeClient(response)
+    backend = FastGPTBackend(client)
+
+    async def fake_aiter_stream_events(actual_response):
+        assert actual_response is response
+        yield SimpleNamespace(
+            kind="answer",
+            data={"choices": [{"delta": {"content": "<state>1002"}}]},
+        )
+        yield SimpleNamespace(
+            kind="flowResponses",
+            data=[
+                {
+                    "moduleName": "文本内容提取事故信息",
+                    "extractResult": {"formUpdate": '{"jdcsl": 2}'},
+                }
+            ],
+        )
+        yield SimpleNamespace(
+            kind="answer",
+            data={"choices": [{"delta": {"content": "</state>你好"}}]},
+        )
+        yield SimpleNamespace(kind="ignored", data={})
+
+    monkeypatch.setattr(
+        "src.backends.fastgpt.aiter_stream_events",
+        fake_aiter_stream_events,
+    )
+
+    events = [
+        event
+        async for event in backend.stream(
+            ChatInput(session_id="session-001", text="你好")
+        )
+    ]
+
+    assert events == [
+        TextDelta("<state>1002"),
+        FormUpdate({"jdcsl": 2}),
+        TextDelta("</state>你好"),
+    ]
+    assert client.calls[0]["stream"] is True