Function chunk_path 

pub fn chunk_path(
    user_id: &str,
    upload_id: &str,
    chunk_index: u32,
) -> Result<ApiPath, UrlError>

Builds the ApiPath addressing a single chunk in the upload collection.

Formats the chunk index as a 5-digit zero-padded number (e.g., 00001). Resulting path: /remote.php/dav/uploads/{user_id}/{upload_id}/{chunk_number}