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/api/test_chat_backend_boundary.py

@@ -0,0 +1,70 @@
+import pytest
+
+from src.api.endpoints import chat
+from src.backends.chat import ChatInput, ChatResult, FormUpdate, TextDelta
+from src.schemas.models import ProcessRequest_chat
+
+
+class FakeBackend:
+    def __init__(self):
+        self.received = []
+
+    async def stream(self, chat_input: ChatInput):
+        self.received.append(chat_input)
+        yield TextDelta("<sta")
+        yield TextDelta("te>1002</state>你")
+        yield FormUpdate({"jdcsl": 2})
+        yield TextDelta("好")
+
+    async def complete(self, chat_input: ChatInput):
+        self.received.append(chat_input)
+        return ChatResult(
+            content="<state>1002</state>你好",
+            status_code="1002",
+            form_update={"jdcsl": 2},
+        )
+
+
+def make_request():
+    return ProcessRequest_chat(
+        sessionId="session-001",
+        timeStamp="20260725120000",
+        text="发生了交通事故",
+        needFormUpdate=True,
+    )
+
+
+@pytest.mark.asyncio
+async def test_non_stream_chat_uses_backend_neutral_result():
+    backend = FakeBackend()
+
+    response = await chat(make_request(), stream=False, backend=backend)
+
+    assert response.outputText == "你好"
+    assert response.nextStageCode == "1002"
+    assert response.formUpdate == {"jdcsl": 2}
+    assert backend.received == [
+        ChatInput(
+            session_id="session-001",
+            text="发生了交通事故",
+            need_form_update=True,
+        )
+    ]
+
+
+@pytest.mark.asyncio
+async def test_stream_chat_keeps_state_prefix_buffering_in_fastapi_layer():
+    backend = FakeBackend()
+
+    response = await chat(make_request(), stream=True, backend=backend)
+    chunks = []
+    async for chunk in response.body_iterator:
+        chunks.append(chunk.decode() if isinstance(chunk, bytes) else chunk)
+    body = "".join(chunks)
+
+    assert body.index("event: stage_code") < body.index("event: text_delta")
+    assert '"nextStageCode": "1002"' in body
+    assert '"text": "你"' in body
+    assert '"text": "好"' in body
+    assert "event: formUpdate" in body
+    assert "event: done" in body