fix: 为FFMPEGVideoLoopFillAudioRequest添加字段验证器

修复video-loop-fill接口无法接受字符串格式S3 URL的问题，添加field_validator来处理video和audio字段的字符串到MediaSource对象的转换

src/BowongModalFunctions/models/requests/models.py

@@ -323,6 +323,26 @@ class FFMPEGVideoLoopFillAudioRequest(BaseFFMPEGTaskRequest):
     video: MediaSource = Field(description="用来填充的视频素材")
     audio: MediaSource = Field(description="被填充的音频素材")
 
+    @field_validator('video', mode='before')
+    @classmethod
+    def parse_video_input(cls, v: Union[str, MediaSource]):
+        if isinstance(v, str):
+            return MediaSource.from_str(v)
+        elif isinstance(v, MediaSource):
+            return v
+        else:
+            raise TypeError(v)
+
+    @field_validator('audio', mode='before')
+    @classmethod
+    def parse_audio_input(cls, v: Union[str, MediaSource]):
+        if isinstance(v, str):
+            return MediaSource.from_str(v)
+        elif isinstance(v, MediaSource):
+            return v
+        else:
+            raise TypeError(v)
+
 
 class FFMPEGExtractFrameRequest(BaseFFMPEGTaskRequest):
     video: MediaSource = Field(description="提取帧画面的来源")