* test(wechat): cover allowed_file_extensions coercion boundaries
* fix(wechat): never widen a configured allowed_file_extensions back to defaults
* fix(wechat): warn when a scalar allowed_file_extensions looks like a list
A comma- or space-separated scalar string is still one literal extension, so
`allowed_file_extensions: ".pdf, .zip"` blocks every file with nothing in the
startup log pointing at the config. Mirror the `_parse_allowed_users` guard.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
