refactor: improve network interface validation

Add context parameter to validInterfacesMap for better error handling and
logging. Move Windows-specific network adapter validation logic to the
ctrld package. Key changes include:

- Add context parameter to validInterfacesMap across all platforms
- Move Windows validInterfaces to ctrld.ValidInterfaces
- Improve error handling for virtual interface detection on Linux
- Update all callers to pass appropriate context

This change improves error reporting and makes the interface validation
code more maintainable across different platforms.

cmd/cli/net_linux.go

@@ -1,12 +1,15 @@
 package cli
 
 import (
+	"context"
 	"net"
 	"net/netip"
 	"os"
 	"strings"
 
 	"tailscale.com/net/netmon"
+
+	"github.com/Control-D-Inc/ctrld"
 )
 
 func patchNetIfaceName(iface *net.Interface) (bool, error) { return true, nil }
@@ -19,16 +22,16 @@ func validInterface(iface *net.Interface, validIfacesMap map[string]struct{}) bo
 }
 
 // validInterfacesMap returns a set containing non virtual interfaces.
-func validInterfacesMap() map[string]struct{} {
+func validInterfacesMap(ctx context.Context) map[string]struct{} {
 	m := make(map[string]struct{})
-	vis := virtualInterfaces()
+	vis := virtualInterfaces(ctx)
 	netmon.ForeachInterface(func(i netmon.Interface, prefixes []netip.Prefix) {
 		if _, existed := vis[i.Name]; existed {
 			return
 		}
 		m[i.Name] = struct{}{}
 	})
-	// Fallback to default route interface if found nothing.
+	// Fallback to the default route interface if found nothing.
 	if len(m) == 0 {
 		defaultRoute, err := netmon.DefaultRoute()
 		if err != nil {
@@ -39,10 +42,15 @@ func validInterfacesMap() map[string]struct{} {
 	return m
 }
 
-// virtualInterfaces returns a map of virtual interfaces on current machine.
-func virtualInterfaces() map[string]struct{} {
+// virtualInterfaces returns a map of virtual interfaces on the current machine.
+func virtualInterfaces(ctx context.Context) map[string]struct{} {
+	logger := ctrld.LoggerFromCtx(ctx)
 	s := make(map[string]struct{})
-	entries, _ := os.ReadDir("/sys/devices/virtual/net")
+	entries, err := os.ReadDir("/sys/devices/virtual/net")
+	if err != nil {
+		logger.Error().Err(err).Msg("failed to read /sys/devices/virtual/net")
+		return nil
+	}
 	for _, entry := range entries {
 		if entry.IsDir() {
 			s[strings.TrimSpace(entry.Name())] = struct{}{}