test: 更新封面 stub 单测，验证 SVG data URI 占位图

- 更新 test_cover_type_manual_with_frame_time：断言 SVG data URI + 手动选帧文字
- 更新 test_cover_type_ai_frame：断言 SVG data URI + 封面生成中文字
- 新增 test_stub_returns_svg_data_uri：验证 stub 不含 /api/v1/ 路径
- 新增 test_manual_stub_returns_svg_data_uri：验证 manual 模式 SVG 占位图

tests/unit/test_shared_ai_service.py

@@ -431,7 +431,8 @@ class TestAiCoverService:
             result = _call_ai_cover_service("plan1", ["a1"], "manual", frame_time=5.5)
         assert result["type"] == "manual"
         assert result["frame_time"] == 5.5
-        assert "5.5" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "手动选帧" in result["image_url"]
 
     def test_cover_type_ai_frame(self):
         with patch("shared.ai_service.time.sleep"):
@@ -440,7 +441,8 @@ class TestAiCoverService:
         assert result["type"] == "ai_frame"
         assert result["frame_time"] == 5.0
         assert result["confidence"] == 0.9
-        assert "plan1" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "封面生成中" in result["image_url"]
 
     def test_cover_type_ai_regenerate(self):
         with patch("shared.ai_service.time.sleep"):
@@ -451,3 +453,20 @@ class TestAiCoverService:
         with patch("shared.ai_service.time.sleep"):
             result = _call_ai_cover_service("plan1", ["a1"], "ai_frame")
         assert 1.0 <= result["frame_time"] <= 10.0
+
+    def test_stub_returns_svg_data_uri(self):
+        """stub 降级返回 SVG data URI，不含任何后端 API 路径."""
+        with patch("shared.ai_service.time.sleep"):
+            result = _call_ai_cover_service("plan1", ["a1"], "ai_frame")
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "/api/v1/" not in result["image_url"]
+        assert "1080" in result["image_url"]
+        assert "1920" in result["image_url"]
+
+    def test_manual_stub_returns_svg_data_uri(self):
+        """manual 模式返回 SVG data URI 占位图."""
+        with patch("shared.ai_service.time.sleep"):
+            result = _call_ai_cover_service("plan1", ["a1"], "manual", frame_time=3.0)
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "/api/v1/" not in result["image_url"]
+        assert "手动选帧" in result["image_url"]