In bt_vrfy.c, the functions __verify_addr_string and __verify_disagg_string (will be added as part of WT-16407) currently always return buf->data, even when formatting work fails internally. This makes failures in debug string construction harder to detect and diagnose.

As a follow up refactor, update these helpers to return an error code when failure occurs and possibly parse string in another way. The goal is to improve error visibility and make debug path behavior more consistent.