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_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