fix(manual): send_message should allow either agent_id or agent_name

Author: Charlotte Zhuang

src/agentex/resources/agents.py

@@ -421,7 +421,7 @@ def send_message(
       extra_body: Body | None = None,
       timeout: float | httpx.Timeout | None | NotGiven = NOT_GIVEN,
     ) -> SendMessageResponse:
-      if agent_id is not None and agent_name is not None:
+      if (agent_id is None) and (agent_name is None):
         raise ValueError("Either agent_id or agent_name must be provided, but not both")
 
       if "stream" in params and params["stream"] == True: