Function read_page_header_len_from_bytes 

pub(crate) fn read_page_header_len_from_bytes(
    context: &SerializedPageReaderContext,
    buffer: &[u8],
    page_index: usize,
    dictionary_page: bool,
) -> Result<(usize, PageHeader)>

Reads (and decrypts, if context carries a crypto context) the page header stored at the front of buffer, returning the header and the number of bytes of buffer it occupies.

This is exposed for callers that need to decode a single page directly from an already-fetched byte range, outside of the normal SerializedPageReader iteration – e.g. decoding a dictionary page standalone.