[client] Support embed.Client on Android with netstack mode (#5623)

* [client] Support embed.Client on Android with netstack mode

embed.Client.Start() calls ConnectClient.Run() which passes an empty
MobileDependency{}. On Android, the engine dereferences nil fields
(IFaceDiscover, NetworkChangeListener, DnsReadyListener) causing panics.

Provide complete no-op stubs so the engine's existing Android code
paths work unchanged — zero modifications to engine.go:

- Add androidRunOverride hook in Run() for Android-specific dispatch
- Add runOnAndroidEmbed() with complete MobileDependency (all stubs)
- Wire default stubs via init() in connect_android_default.go:
  noopIFaceDiscover, noopNetworkChangeListener, noopDnsReadyListener
- Forward logPath to c.run()

Tested: embed.Client starts on Android arm64, joins mesh via relay,
discovers peers, localhost proxy works for TCP+UDP forwarding.

* [client] Fix TestServiceParamsPath for Windows path separators

Use filepath.Join in test assertions instead of hardcoded POSIX paths
so the test passes on Windows where filepath.Join uses backslashes.

client/internal/connect.go

@@ -44,6 +44,10 @@ import (
 	"github.com/netbirdio/netbird/version"
 )
 
+// androidRunOverride is set on Android to inject mobile dependencies
+// when using embed.Client (which calls Run() with empty MobileDependency).
+var androidRunOverride func(c *ConnectClient, runningChan chan struct{}, logPath string) error
+
 type ConnectClient struct {
 	ctx            context.Context
 	config         *profilemanager.Config
@@ -76,6 +80,9 @@ func (c *ConnectClient) SetUpdateManager(um *updater.Manager) {
 
 // Run with main logic.
 func (c *ConnectClient) Run(runningChan chan struct{}, logPath string) error {
+	if androidRunOverride != nil {
+		return androidRunOverride(c, runningChan, logPath)
+	}
 	return c.run(MobileDependency{}, runningChan, logPath)
 }
 

client/internal/connect_android_default.go

@@ -0,0 +1,73 @@
+//go:build android
+
+package internal
+
+import (
+	"net/netip"
+
+	"github.com/netbirdio/netbird/client/internal/dns"
+	"github.com/netbirdio/netbird/client/internal/listener"
+	"github.com/netbirdio/netbird/client/internal/stdnet"
+)
+
+// noopIFaceDiscover is a stub ExternalIFaceDiscover for embed.Client on Android.
+// It returns an empty interface list, which means ICE P2P candidates won't be
+// discovered — connections will fall back to relay. Applications that need P2P
+// should provide a real implementation via runOnAndroidEmbed that uses
+// Android's ConnectivityManager to enumerate network interfaces.
+type noopIFaceDiscover struct{}
+
+func (noopIFaceDiscover) IFaces() (string, error) {
+	// Return empty JSON array — no local interfaces advertised for ICE.
+	// This is intentional: without Android's ConnectivityManager, we cannot
+	// reliably enumerate interfaces (netlink is restricted on Android 11+).
+	// Relay connections still work; only P2P hole-punching is disabled.
+	return "[]", nil
+}
+
+// noopNetworkChangeListener is a stub for embed.Client on Android.
+// Network change events are ignored since the embed client manages its own
+// reconnection logic via the engine's built-in retry mechanism.
+type noopNetworkChangeListener struct{}
+
+func (noopNetworkChangeListener) OnNetworkChanged(string) {
+	// No-op: embed.Client relies on the engine's internal reconnection
+	// logic rather than OS-level network change notifications.
+}
+
+func (noopNetworkChangeListener) SetInterfaceIP(string) {
+	// No-op: in netstack mode, the overlay IP is managed by the userspace
+	// network stack, not by OS-level interface configuration.
+}
+
+// noopDnsReadyListener is a stub for embed.Client on Android.
+// DNS readiness notifications are not needed in netstack/embed mode
+// since system DNS is disabled and DNS resolution happens externally.
+type noopDnsReadyListener struct{}
+
+func (noopDnsReadyListener) OnReady() {
+	// No-op: embed.Client does not need DNS readiness notifications.
+	// System DNS is disabled in netstack mode.
+}
+
+var _ stdnet.ExternalIFaceDiscover = noopIFaceDiscover{}
+var _ listener.NetworkChangeListener = noopNetworkChangeListener{}
+var _ dns.ReadyListener = noopDnsReadyListener{}
+
+func init() {
+	// Wire up the default override so embed.Client.Start() works on Android
+	// with netstack mode. Provides complete no-op stubs for all mobile
+	// dependencies so the engine's existing Android code paths work unchanged.
+	// Applications that need P2P ICE or real DNS should replace this by
+	// setting androidRunOverride before calling Start().
+	androidRunOverride = func(c *ConnectClient, runningChan chan struct{}, logPath string) error {
+		return c.runOnAndroidEmbed(
+			noopIFaceDiscover{},
+			noopNetworkChangeListener{},
+			[]netip.AddrPort{},
+			noopDnsReadyListener{},
+			runningChan,
+			logPath,
+		)
+	}
+}

client/internal/connect_android_embed.go

@@ -0,0 +1,32 @@
+//go:build android
+
+package internal
+
+import (
+	"net/netip"
+
+	"github.com/netbirdio/netbird/client/internal/dns"
+	"github.com/netbirdio/netbird/client/internal/listener"
+	"github.com/netbirdio/netbird/client/internal/stdnet"
+)
+
+// runOnAndroidEmbed is like RunOnAndroid but accepts a runningChan
+// so embed.Client.Start() can detect when the engine is ready.
+// It provides complete MobileDependency so the engine's existing
+// Android code paths work unchanged.
+func (c *ConnectClient) runOnAndroidEmbed(
+	iFaceDiscover stdnet.ExternalIFaceDiscover,
+	networkChangeListener listener.NetworkChangeListener,
+	dnsAddresses []netip.AddrPort,
+	dnsReadyListener dns.ReadyListener,
+	runningChan chan struct{},
+	logPath string,
+) error {
+	mobileDependency := MobileDependency{
+		IFaceDiscover:         iFaceDiscover,
+		NetworkChangeListener: networkChangeListener,
+		HostDNSAddresses:      dnsAddresses,
+		DnsReadyListener:      dnsReadyListener,
+	}
+	return c.run(mobileDependency, runningChan, logPath)
+}