resolved: skip cache flush on server switch/re-probe when StaleRetentionSec is set

manager_set_dns_server() and dns_server_flush_cache() call dns_cache_flush()
unconditionally, wiping the entire cache even when StaleRetentionSec is
configured. This defeats serve-stale by discarding cached records that should
remain available during server switches and feature-level re-probes.

The original serve-stale commit (5ed91481ab) added a stale_retention_usec
guard to link_set_dns_server(), and a later commit (7928c0e0a1) added the
same guard to dns_delegate_set_dns_server(), but these two call sites in
resolved-dns-server.c were missed.

This is particularly visible with DNSOverTLS, where TLS handshake failures
trigger frequent feature-level downgrades and re-probes via
dns_server_flush_cache(), flushing the cache each time.

Add the same stale_retention_usec guard to both call sites so that cache
entries are allowed to expire naturally via dns_cache_prune() when
serve-stale is enabled.

Fixes: #40781

This commit was prepared with assistance from an AI coding agent (GitHub
Copilot). All changes have been reviewed for correctness and adherence to the
systemd coding style.

src/resolve/resolved-dns-server.c

@@ -1036,7 +1036,8 @@ DnsServer *manager_set_dns_server(Manager *m, DnsServer *s) {
         dns_server_unref(m->current_dns_server);
         m->current_dns_server = dns_server_ref(s);
 
-        if (m->unicast_scope)
+        /* Skip flushing the cache if server stale feature is enabled. */
+        if (m->unicast_scope && m->stale_retention_usec == 0)
                 dns_cache_flush(&m->unicast_scope->cache);
 
         (void) manager_send_changed(m, "CurrentDNSServer");
@@ -1155,6 +1156,10 @@ void dns_server_flush_cache(DnsServer *s) {
         if (!scope)
                 return;
 
+        /* Skip flushing the cache if server stale feature is enabled. */
+        if (s->manager->stale_retention_usec > 0)
+                return;
+
         dns_cache_flush(&scope->cache);
 }