feat: add stateful responses API

backend/services/brains/dify_brain.py

@@ -24,6 +24,8 @@ class DifyBrain(BaseBrain):
     def __init__(self):
         self._user_id = f"ai-video-{uuid4().hex}"
         self._client: AsyncClient | None = None
+        self._llm: DifyLLMService | None = None
+        self._restored_conversation_id = ""
 
     def _get_client(self, cfg: AssistantConfig) -> AsyncClient:
         if self._client is None:
@@ -54,8 +56,25 @@ class DifyBrain(BaseBrain):
             return cfg.greeting
 
     def build_llm(self, cfg: AssistantConfig, context: LLMContext) -> FrameProcessor:
-        return DifyLLMService(
+        self._llm = DifyLLMService(
             cfg,
             client=self._get_client(cfg),
             user_id=self._user_id,
+            conversation_id=self._restored_conversation_id,
         )
+        return self._llm
+
+    def restore_text_state(self, snapshot: dict) -> None:
+        if snapshot.get("type") != "dify":
+            raise ValueError("Responses runtime state does not belong to Dify")
+        self._user_id = str(snapshot.get("user_id") or self._user_id)
+        self._restored_conversation_id = str(snapshot.get("conversation_id") or "")
+
+    def text_state_snapshot(self) -> dict:
+        return {
+            "type": "dify",
+            "user_id": self._user_id,
+            "conversation_id": (
+                self._llm.conversation_id if self._llm is not None else ""
+            ),
+        }