fix: dev: Preserve the request message across async SIG(0) processing

For SIG(0)-signed requests, view matching is offloaded and the request
is finished asynchronously from ns_client_request_continue(), which
passes client->inner.buffer to dns_dt_send().  That buffer aliases the
network manager's receive buffer, only valid during the read callback,
so it may already be freed and reused, producing garbage dnstap frames
(e.g. the "upforwd" sig0-over-DoT test fails with UQ=0).

Copy the request message when entering async mode and reference the
copy, freeing it in ns__client_reset_cb().

Assisted-by: Claude:claude-opus-4-8
Closes #6139

Merge branch '6139-dnstap-sig0-request-buffer-uaf' into 'main'

See merge request isc-projects/bind9!12189