fix issue wrong agent prologue for api (#2246)

### What problem does this PR solve?

#2242
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

agent/canvas.py

                 pat = " => ".join([p.split(":")[0] for p in path[0:l]])
                 return pat + " => " + pat
 
-        return False
+        return
+
+    def get_prologue(self):
+        return self.components["begin"]["obj"]._param.prologue

api/apps/api_app.py

     req = request.json
     try:
         if objs[0].source == "agent":
-            e, c = UserCanvasService.get_by_id(objs[0].dialog_id)
+            e, cvs = UserCanvasService.get_by_id(objs[0].dialog_id)
             if not e:
                 return server_error_response("canvas not found.")
+            if not isinstance(cvs.dsl, str):
+                cvs.dsl = json.dumps(cvs.dsl, ensure_ascii=False)
+            canvas = Canvas(cvs.dsl, objs[0].tenant_id)
             conv = {
                 "id": get_uuid(),
-                "dialog_id": c.id,
+                "dialog_id": cvs.id,
                 "user_id": request.args.get("user_id", ""),
-                "message": [{"role": "assistant", "content": "Hi there!"}],
+                "message": [{"role": "assistant", "content": canvas.get_prologue()}],
                 "source": "agent"
             }
             API4ConversationService.save(**conv)