fix(test): mock run_ffmpeg to create output file in cover frame tests

tests/unit/test_cover_frame_pre_extract.py

@@ -22,6 +22,9 @@ class TestExtractFirstFrame(unittest.TestCase):
         """默认在视频 15% 处抽帧."""
         from video_processing.thumbnail_generator import extract_first_frame
 
+        # Mock run_ffmpeg 创建输出文件（ffmpeg 真实行为）
+        mock_run.side_effect = lambda cmd, **kw: Path(cmd[-1]).write_bytes(b"\xff\xd8\xff\xe0fake")
+
         with tempfile.NamedTemporaryFile(suffix=".mp4") as video:
             Path(video.name).write_bytes(b"fake video")
             result = extract_first_frame(video.name)
@@ -40,6 +43,8 @@ class TestExtractFirstFrame(unittest.TestCase):
         """自定义抽帧位置."""
         from video_processing.thumbnail_generator import extract_first_frame
 
+        mock_run.side_effect = lambda cmd, **kw: Path(cmd[-1]).write_bytes(b"\xff\xd8\xff\xe0fake")
+
         with tempfile.NamedTemporaryFile(suffix=".mp4") as video:
             Path(video.name).write_bytes(b"fake video")
             result = extract_first_frame(video.name, seek_ratio=0.5)
@@ -77,6 +82,8 @@ class TestExtractFirstFrame(unittest.TestCase):
         """默认保持原始分辨率（width=-1, height=-1）."""
         from video_processing.thumbnail_generator import extract_first_frame
 
+        mock_run.side_effect = lambda cmd, **kw: Path(cmd[-1]).write_bytes(b"\xff\xd8\xff\xe0fake")
+
         with tempfile.NamedTemporaryFile(suffix=".mp4") as video:
             Path(video.name).write_bytes(b"fake video")
             result = extract_first_frame(video.name)
@@ -95,6 +102,8 @@ class TestExtractFirstFrame(unittest.TestCase):
         """指定宽度时添加 scale 滤镜."""
         from video_processing.thumbnail_generator import extract_first_frame
 
+        mock_run.side_effect = lambda cmd, **kw: Path(cmd[-1]).write_bytes(b"\xff\xd8\xff\xe0fake")
+
         with tempfile.NamedTemporaryFile(suffix=".mp4") as video:
             Path(video.name).write_bytes(b"fake video")
             result = extract_first_frame(video.name, width=640)