Function extract_postgres_lsn_from_offset_str 

pub fn extract_postgres_lsn_from_offset_str(offset_str: &str) -> Option<u64>

Extract PostgreSQL LSN value from a CDC offset JSON string.

This is a standalone helper function that can be used when you only have the offset string (e.g., in callbacks) and don’t have access to the Split object.

Returns Some(lsn) if the LSN is found and can be parsed as u64, None otherwise.