Function apply_app_config_to_yaml_file 

pub(crate) fn apply_app_config_to_yaml_file(
    path: &Path,
    target: &Value,
) -> Result<String>

Convenience wrapper: read the original text from path, apply target onto it, and return the format-preserved result. If the file does not exist or cannot be read, falls back to a plain serialization of target.