[banghamid]

Description
When form_dropdown() or form_multiselect() is called without an explicit $selected argument, form_dropdown() attempts to infer the selected option(s) from $_POST via service('superglobals')->post().

Previously:

1. When the $_POST entry for the field was an array (for example, from a multiple-select submission), $selected = [$postValue] wrapped the array inside another array ([['volvo', 'audi']]), causing the subsequent (string) $item cast to throw an unhandled ErrorException: Array to string conversion.
2. When field names used POST array notation (e.g., shirts[] as required by form_multiselect()), post('shirts[]') did not strip the trailing [] to look up $_POST['shirts'].

This PR:

• Preserves array $postValue values via is_array($postValue) ? $postValue : [$postValue] instead of wrapping them in a nested array.
• Strips trailing [] from the field name when looking up the inferred value in $_POST.
• Adds regression tests in FormHelperTest (testFormDropdownInferredArray() and testFormMultiselectInferred()).

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide