Support valueless HTML attributes in rendered markup

[Copilot]

Rendering currently requires key="value" for every attribute, which prevents valid HTML boolean/valueless attributes like required. This update allows emitting attributes without values while preserving safe attribute rendering.

• Attribute rendering behavior

  • Updated wrapper attribute serialization in Markup::wrapperOpenerTag():
    • '' (empty string) now renders as a valueless attribute (required)
    • non-empty values continue rendering as key="value"

• Output safety

  • Escaped non-empty attribute values at render time with htmlspecialchars(..., ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8') before interpolation.

• Docs

  • Added a setAttribute('required', '') example in docs/markup.md to document boolean/valueless attribute usage.

$input = Markup::make('input', attributes: [
    'required' => '',
    'name' => 'email',
]);

echo $input->render();
// <input required name="email"></input>