[9.20] chg: dev: Implement Fisher-Yates shuffle for nameserver selection

Replace the two-pass "random start index and wrap around" logic in
fctx_getaddresses_nameservers() with a statistically sound partial
Fisher-Yates shuffle.

The previous implementation picked a random starting node and did two
passes over the linked list to find query candidates. The new logic
introduces fctx_getaddresses_nsorder() to perform an in-place
randomization of indices into a bounded, stack-allocated lookup array
(nsorder) representing the "winning" fetch slots.

The nameserver dataset is now traversed in exactly one sequential pass:
1. Every nameserver is evaluated for local cached data.
2. If the current nameserver's sequential index exists in the randomized
   nsorder array, it is permitted to launch an outgoing network fetch.
3. If not, it is restricted to local lookups via DNS_ADBFIND_NOFETCH.

This guarantees a fair random distribution for outbound queries while
maximizing local cache hits, entirely within O(1) memory and without
the overhead of linked-list pointer shuffling or dynamic allocation.

Closes #5695

Backport of MR !11604

Merge branch 'backport-5695-refactor-the-random-NS-selection-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11606