volume: pin validated copy/tail source addresses at dial time

validateReplicaTarget resolves the source hostname once, but the gRPC
client resolved it again at connect, leaving a DNS-rebinding window for
hostname sources. The copy and tail source dials now run through the
same guardedDialerPolicy the remote-storage path uses, so every resolved
address is re-checked against the replica deny list (private peers
allowed) immediately before the TCP connect. guardedDialerPolicy also
moves to util.OutboundDialContext so the guarded path keeps the -ip.bind
source binding the default gRPC dialer had.

The Rust volume server mirrors this with connect_guarded, a tonic
connector that resolves, re-checks each address, and connects to the
first passing IP; handlers use it whenever the untrusted-endpoint
opt-out is off. A handler-level test now exercises the enabled
validation branches for all three source-taking RPCs.

weed/shell/command_volume_merge.go

@@ -232,14 +232,14 @@ func startTailNeedleStream(grpcDialOption grpc.DialOption, volumeId needle.Volum
 	ch := make(chan *needle.Needle, 32)
 	stream := &tailNeedleStream{ch: ch}
 	go func() {
-		err := operation.TailVolumeFromSource(server, grpcDialOption, volumeId, 0, mergeIdleTimeoutSeconds, func(n *needle.Needle) error {
+		err := operation.TailVolumeFromSource(server, volumeId, 0, mergeIdleTimeoutSeconds, func(n *needle.Needle) error {
 			select {
 			case ch <- n:
 			case <-done:
 				return fmt.Errorf("merge cancelled")
 			}
 			return nil
-		})
+		}, grpcDialOption)
 		close(ch)
 		stream.setErr(err)
 	}()