fix(dialer): add switchNetns stubs for all non-Linux platforms

The switchNetns function was only defined for linux (dialer_netns.go,
build tag linux && !android) and android (dialer_android.go, stub).
This left darwin, freebsd, openbsd, windows, and other platforms with
an undefined symbol at dialer.go:55.

Add no-op stubs to all remaining platform files so the dialer package
compiles on every GOOS.

internal/net/dialer/dialer_darwin.go

@@ -35,3 +35,8 @@ func bindDevice(network, address string, fd uintptr, ifceName string) error {
 func setMark(fd uintptr, mark int) error {
 	return nil
 }
+
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}

internal/net/dialer/dialer_freebsd.go

@@ -14,3 +14,8 @@ func setMark(fd uintptr, mark int) error {
 	}
 	return unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_USER_COOKIE, mark)
 }
+
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}

internal/net/dialer/dialer_openbsd.go

@@ -14,3 +14,8 @@ func setMark(fd uintptr, mark int) error {
 	}
 	return unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_RTABLE, mark)
 }
+
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}

internal/net/dialer/dialer_other.go

@@ -9,3 +9,8 @@ func bindDevice(network, address string, fd uintptr, ifceName string) error {
 func setMark(fd uintptr, mark int) error {
 	return nil
 }
+
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}

internal/net/dialer/dialer_windows.go

@@ -49,6 +49,11 @@ func setMark(fd uintptr, mark int) error {
 	return nil
 }
 
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}
+
 func bindSocketToInterface4(handle windows.Handle, index uint32) error {
 	// For IPv4, this parameter must be an interface index in network byte order.
 	// Ref: https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options