TUN inbound: IPv6 entries in dns keep the TUN from starting on Windows when IPv6 cannot be set up, as in gateway and autoSystemRoutingTable

Follow-up to #7325: an IPv6 server in dns now asks for IPv6 as well.
When the adapter has no IPv6 interface, or refuses its IPv6 DNS servers,
the TUN fails to start instead of starting without them. Without IPv6 in
any of the three, both are still skipped: the IPv6 DNS servers that Xray
sets on every start are none then.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

proxy/tun/README.md

@@ -223,7 +223,7 @@ To make it start, wintun.dll specific for your Windows/arch must be present next
 
 After the start network adapter with the name you chose in the config will be created in the system, and exist while Xray is running. A Wintun adapter that already has that name is reused.
 
-With IPv6 disabled system-wide (the `DisabledComponents` registry value), the adapter has no IPv6, and the TUN starts with IPv4 only, unless `gateway` or `autoSystemRoutingTable` has IPv6 entries, which then keep it from starting.
+With IPv6 disabled system-wide (the `DisabledComponents` registry value), the adapter has no IPv6, and the TUN starts with IPv4 only, unless `gateway`, `dns` or `autoSystemRoutingTable` has IPv6 entries, which then keep it from starting.
 
 When `dns` is set, those servers are applied to the adapter. Windows is kept from registering the TUN's addresses in DNS, and its DNS cache is flushed when the TUN starts and stops. Xray's own lookups through the system's DNS servers, like those of the `localhost` DNS server with `autoOutboundsInterface`, skip them, unless another interface uses them as well, as they would lead back into the TUN.
 

proxy/tun/tun_windows.go

@@ -115,6 +115,7 @@ func (t *WindowsTun) Start() (err error) {
 	for _, ip := range t.options.DNS {
 		dns = append(dns, netip.MustParseAddr(ip))
 	}
+	dns6 := slices.ContainsFunc(dns, netip.Addr.Is6)
 
 	var route4, route6 bool
 	routesMap := make(map[winipcfg.RouteData]struct{})
@@ -175,7 +176,7 @@ startOver:
 		if err != nil {
 			// With IPv6 disabled system-wide (DisabledComponents), the adapter has no
 			// IPv6 interface at all. Skip the family unless the config asks for it.
-			if err == windows.ERROR_NOT_FOUND && family == windows.AF_INET6 && !address6 && !route6 {
+			if err == windows.ERROR_NOT_FOUND && family == windows.AF_INET6 && !address6 && !route6 && !dns6 {
 				continue
 			}
 			return err
@@ -204,7 +205,7 @@ startOver:
 			// Skip IPv6 the config does not ask for also when setting its DNS
 			// servers fails, as reported on Windows 11 24H2 with IPv6 disabled
 			// (Access is denied).
-			if family == windows.AF_INET6 && !address6 && !route6 {
+			if family == windows.AF_INET6 && !address6 && !route6 && !dns6 {
 				continue
 			}
 			firstErr = errors.New("unable to set DNS").Base(err)