Feat/add support for forcing device auth flow on ios (#4944)

* updates to client file writing

* numerous

* minor

* - Align OnLoginSuccess behavior with Android (only call on nil error)
- Remove verbose debug logging from WaitToken in device_flow.go
- Improve TUN FD=0 fallback comments and warning messages
- Document why config save after login differs from Android

* Add nolint directive for staticcheck SA1029 in login.go

* Fix CodeRabbit review issues for iOS/tvOS SDK

- Remove goroutine from OnLoginSuccess callback, invoke synchronously
- Stop treating PermissionDenied as success, propagate as permanent error
- Replace context.TODO() with bounded timeout context (30s) in RequestAuthInfo
- Handle DirectUpdateOrCreateConfig errors in IsLoginRequired and LoginForMobile
- Add permission enforcement to DirectUpdateOrCreateConfig for existing configs
- Fix variable shadowing in device_ios.go where err was masked by := in else block

* Address additional CodeRabbit review issues for iOS/tvOS SDK

- Make tunFd == 0 a hard error with exported ErrInvalidTunnelFD (remove dead fallback code)
- Apply defaults in ConfigFromJSON to prevent partially-initialized configs
- Add nil guards for listener/urlOpener interfaces in public SDK entry points
- Reorder config save before OnLoginSuccess to prevent teardown race
- Add explanatory comment for urlOpener.Open goroutine

* Make urlOpener.Open() synchronous in device auth flow

client/iface/device/device_ios.go

@@ -4,6 +4,7 @@
 package device
 
 import (
+	"fmt"
 	"os"
 
 	log "github.com/sirupsen/logrus"
@@ -45,10 +46,31 @@ func NewTunDevice(name string, address wgaddr.Address, port int, key string, mtu
 	}
 }
 
+// ErrInvalidTunnelFD is returned when the tunnel file descriptor is invalid (0).
+// This typically means the Swift code couldn't find the utun control socket.
+var ErrInvalidTunnelFD = fmt.Errorf("invalid tunnel file descriptor: fd is 0 (Swift failed to locate utun socket)")
+
 func (t *TunDevice) Create() (WGConfigurer, error) {
 	log.Infof("create tun interface")
 
-	dupTunFd, err := unix.Dup(t.tunFd)
+	var tunDevice tun.Device
+	var err error
+
+	// Validate the tunnel file descriptor.
+	// On iOS/tvOS, the FD must be provided by the NEPacketTunnelProvider.
+	// A value of 0 means the Swift code couldn't find the utun control socket
+	// (the low-level APIs like ctl_info, sockaddr_ctl may not be exposed in
+	// tvOS SDK headers). This is a hard error - there's no viable fallback
+	// since tun.CreateTUN() cannot work within the iOS/tvOS sandbox.
+	if t.tunFd == 0 {
+		log.Errorf("Tunnel file descriptor is 0 - Swift code failed to locate the utun control socket. " +
+			"On tvOS, ensure the NEPacketTunnelProvider is properly configured and the tunnel is started.")
+		return nil, ErrInvalidTunnelFD
+	}
+
+	// Normal iOS/tvOS path: use the provided file descriptor from NEPacketTunnelProvider
+	var dupTunFd int
+	dupTunFd, err = unix.Dup(t.tunFd)
 	if err != nil {
 		log.Errorf("Unable to dup tun fd: %v", err)
 		return nil, err
@@ -60,7 +82,7 @@ func (t *TunDevice) Create() (WGConfigurer, error) {
 		_ = unix.Close(dupTunFd)
 		return nil, err
 	}
-	tunDevice, err := tun.CreateTUNFromFile(os.NewFile(uintptr(dupTunFd), "/dev/tun"), 0)
+	tunDevice, err = tun.CreateTUNFromFile(os.NewFile(uintptr(dupTunFd), "/dev/tun"), 0)
 	if err != nil {
 		log.Errorf("Unable to create new tun device from fd: %v", err)
 		_ = unix.Close(dupTunFd)