Same work as WT-18810, but for the ones mentioned in WT-18039:

• This warning:

      if (orig < size)
          /* FIXME-WT-18039: Replace this and the WT_ASSERT below with a WT_ASSERT_ALWAYS. */
          __wt_verbose_warning(session, WT_VERB_DISAGGREGATED_STORAGE,
            "disaggregated block size underflow: decrementing %" PRIu64 " from %" PRIu64
            ", clamped to 0",
            size, orig);
  

• This error:

              if (db < sub || db - sub < WT_DISAGG_CHECKPOINT_SIZE_BUFFER) {
                  __wt_verbose_error(session, WT_VERB_DISAGGREGATED_STORAGE,
                    "disaggregated database size would fall below the checkpoint buffer: "
                    "decrementing %" PRIu64 " from %" PRIu64
                    ", clamped to the checkpoint size buffer %" PRIu64,
                    sub, db, (uint64_t)WT_DISAGG_CHECKPOINT_SIZE_BUFFER);
                  new_size = WT_DISAGG_CHECKPOINT_SIZE_BUFFER;