Reject oversized RRsets at slab construction

Reject oversized RRsets at slab construction

dns_rdataslab_fromrdataset(), dns_rdataslab_merge() and
dns_rdataslab_subtract() summed per-record storage into an
unsigned int with no upper-bound check.  An RRset whose total
encoded size exceeds DNS_RDATA_MAXLENGTH cannot fit in a DNS
message and is unservable; building its in-memory representation
only burns memory on data that will fail at response time, and at
the upper bound the running sum could in theory wrap.

Cap the running total at DNS_RDATA_MAXLENGTH and return ISC_R_NOSPACE
when exceeded.  Update the qpdb cache memory-purge test to use a
record size that fits within the new limit.

Assisted-by: Claude:claude-opus-4-7
(cherry picked from commit f9d24b1b8548363b73a086de9d7038ac2517cd12)