test: 修复 test_mediakit_cover 中 placeholder 断言，适配 SVG data URI 占位图

tests/unit/test_mediakit_cover.py

@@ -185,7 +185,7 @@ class TestAICoverService:
 
         # 应该降级到 stub
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     def test_call_ai_cover_without_video_url_fallback(self):
         """没有视频 URL 时使用 stub."""
@@ -199,7 +199,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     def test_call_ai_cover_upload_type(self):
         """upload 类型直接返回."""
@@ -247,7 +247,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     @patch("packages.shared.mediakit_client.get_mediakit_client")
     def test_call_ai_cover_empty_frames_fallback(self, mock_get_client):
@@ -267,7 +267,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
 
 class TestGenerateCover: