fix: make showHidden(false) permanently hide hidden options - #2583

Fixes #2449.

.showHidden(false) currently only skips registering the --show-hidden flag — it never stops it from working. addShowHiddenOpt() returns early, so options.showHiddenOpt keeps its default 'show-hidden' value, and filterHiddenOptions() reveals a hidden key whenever argv[showHiddenOpt] is set:

yargs.getOptions().hiddenOptions.indexOf(key) < 0 ||
  (yargs.parsed as DetailedArguments).argv[yargs.getOptions().showHiddenOpt]

The result is what @shadowspawn described in the issue: --show-hidden is absent from the help output, but passing it alongside --help still lists every hidden option.

yargs('--help --show-hidden')
  .options({secret: {describe: 'a secret', hidden: true}})
  .showHidden(false)
  .parse()
// --secret is still listed

This clears showHiddenOpt when the option is disabled, so there is no name left for filterHiddenOptions() to look up.

On the "bug or documentation problem?" question from the issue — docs/api.md is already unambiguous about the intent, so I treated it as a code bug and left the docs alone:

If the first argument is a boolean, it enables/disables this option altogether. i.e. hidden keys will be permanently hidden if first argument is false.

Unchanged: the default behaviour (--show-hidden works without calling .showHidden()), .showHidden(), and .showHidden('custom-name').

Two tests added, both failing before the change — a help-output test next to the existing --show-hidden cases in test/usage.mjs, and an option-state test in the showHidden block of test/yargs.mjs. npm run check is clean. npm test goes from 821 to 823 passing; the 18 failures I see locally are the commandDir ESM and integration suites, and they fail identically on an unmodified main in my environment.