fix(forward): prevent busy loop on connection err (#7704)

Fixes a bug in the forward plugin where an immediate connection
failure (e.g., TCP RST) could trigger an infinite busy loop. The
retry logic failed to increment the "fails" counter when a
connection error occurred, causing the loop condition to
remain permanently true. This patch fixes it and adds a
regression test.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/forward/forward.go

@@ -191,6 +191,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 				proxy.Healthcheck()
 			}
 
+			fails++
 			if fails < len(f.proxies) {
 				continue
 			}