fix: preserve explicit zero values in native relay requests

relay/channel/codex/adaptor.go

@@ -102,7 +102,7 @@ func (a *Adaptor) ConvertOpenAIResponsesRequest(c *gin.Context, info *relaycommo
 	// codex: store must be false
 	request.Store = json.RawMessage("false")
 	// rm max_output_tokens
-	request.MaxOutputTokens = 0
+	request.MaxOutputTokens = nil
 	request.Temperature = nil
 	return request, nil
 }