test(vision): 补 L3 空 delta 流端到端回归

execute_streaming_errors_when_model_returns_empty_delta_stream:
mock 端点返回 delta-less data 事件 + [DONE],断言统一报
'no usable content' 且提示 empty delta stream,锁定
saw_data_event=true + 空内容的报错分支(原单测仅覆盖
process_sse_line 解析,未走 execute_streaming 判定)。

vision::tools 23 个用例全过。

Author: luzeyang (INT)

crates/tui/src/vision/tools.rs

@@ -1031,6 +1031,32 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn execute_streaming_errors_when_model_returns_empty_delta_stream() {
+        // L3 回归:saw_data_event=true 但所有 delta 均无 content(模型返空),
+        // 与端点忽略 stream 参数统一报 "no usable content",不再静默返回空
+        // 成功;提示语指明 empty delta stream 以区分两种成因。
+        let body = "data: {\"choices\":[{\"delta\":{}}]}\n\ndata: [DONE]\n\n".to_string();
+        let (base_url, _request_rx) = serve_once("HTTP/1.1 200 OK", body).await;
+        let (_tmp, ctx) = workspace_with_image();
+        let mut config = fake_config();
+        config.base_url = Some(base_url);
+        let tool = ImageAnalyzeTool::new(config);
+
+        let err = tool
+            .execute(json!({"image_path": "img.png"}), &ctx)
+            .await
+            .expect_err("empty delta stream must error instead of returning empty success");
+        assert!(
+            err.to_string().contains("no usable content"),
+            "error must call out the unified 'no usable content' message; got {err}"
+        );
+        assert!(
+            err.to_string().contains("empty delta stream"),
+            "error must hint at the model returning empty deltas; got {err}"
+        );
+    }
+
     #[tokio::test]
     async fn execute_non_streaming_parses_plain_json_response() {
         let body =