Add PG18 API compatibility.
Use process_shared_preload_libraries_in_progress instead of parsing the GUC string with GetConfigOption/strstr.
Add ProcessUtility hook to block DDL in read-only mode
Use XactReadOnly via GUC machinery instead of manual command filtering.
Add walk_plan() that recursively checks targetlists and quals of the entire plan tree for volatile functions using the built-in contain_volatile_functions()