fix(feishu): convert audio type to file for API compatibility

Feishu's GetMessageResource API only accepts 'image' or 'file' as the
type parameter. When downloading voice messages, nanobot was passing
'audio' which caused the API to reject the request with an error.

This fix converts 'audio' to 'file' in _download_file_sync method
before making the API call, allowing voice messages to be downloaded
and transcribed successfully.

Fixes voice message download failure in Feishu channel.

nanobot/channels/feishu.py

@@ -530,6 +530,10 @@ class FeishuChannel(BaseChannel):
         self, message_id: str, file_key: str, resource_type: str = "file"
     ) -> tuple[bytes | None, str | None]:
         """Download a file/audio/media from a Feishu message by message_id and file_key."""
+        # Feishu API only accepts 'image' or 'file' as type parameter
+        # Convert 'audio' to 'file' for API compatibility
+        if resource_type == "audio":
+            resource_type = "file"
         try:
             request = (
                 GetMessageResourceRequest.builder()