fix: dev: Fix recognition of DNSSEC keys using the PRIVATEDNS private algorithm

`dst_algorithm_fromprivatedns()` parses the algorithm name with `dns_name_fromwire()`,
which reads only the buffer's active region; the callers in frombuffer() and the
resolver never set it, so PRIVATEDNS keys were always rejected as unsupported.

Merge branch '5871-followup-fix-privatedns-algorithm-extraction' into 'main'

See merge request isc-projects/bind9!12485