fix: Create conversation sessions will lost prologue (#9666)

### What problem does this PR solve?

When create conversation,the prologue hasn't save in conversation.

### Type of change

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

api/apps/sdk/session.py

@@ -84,18 +84,10 @@ def create_agent_session(tenant_id, agent_id):
     session_id=get_uuid()
     canvas = Canvas(cvs.dsl, tenant_id, agent_id)
     canvas.reset()
-    conv = {
-        "id": session_id,
-        "dialog_id": cvs.id,
-        "user_id": user_id,
-        "message": [],
-        "source": "agent",
-        "dsl": cvs.dsl
-    }
-    API4ConversationService.save(**conv)
     
     cvs.dsl = json.loads(str(canvas))
     conv = {"id": session_id, "dialog_id": cvs.id, "user_id": user_id, "message": [{"role": "assistant", "content": canvas.get_prologue()}], "source": "agent", "dsl": cvs.dsl}
+    API4ConversationService.save(**conv)
     conv["agent_id"] = conv.pop("dialog_id")
     return get_result(data=conv)