Function try_shorten_block_smallest_key 

pub fn try_shorten_block_smallest_key(
    prev_block_last: &FullKey<&[u8]>,
    block_smallest: &FullKey<&[u8]>,
) -> Option<FullKey<Vec<u8>>>

Attempts to shorten block_smallest key while preserving block boundary correctness.

Returns a shortened key if prev_block_last < shortened <= block_smallest can be satisfied with fewer bytes. This reduces block metadata size by exploiting common prefixes between adjacent blocks.

Returns None if the key cannot be shortened (e.g., keys differ only in the last byte).