Add label_for option to wrappers

[55728]

Closes #1800

When building custom wrappers with tag: :label, there was no way to automatically add the for attribute pointing to the corresponding input element.

This PR adds a label_for option to the wrapper DSL. When set to true, the wrapper resolves the input's id and adds it as the for attribute on the wrapping element.

Usage

config.wrappers :custom_label do |b|
  b.wrapper tag: :label, label_for: true, class: "custom-label" do |c|
    c.use :input
    c.wrapper tag: :div do |d|
      d.use :hint
    end
  end
end

This produces:

<label for="user_name" class="custom-label">
  <input type="text" id="user_name" />
  <div>
    <span class="hint">...</span>
  </div>
</label>

How it works

• label_for: true is opt-in — existing wrappers are not affected
• If input_html: { id: "custom_id" } is provided, that value is used
• Otherwise, field_id (Rails 7.0+) generates the standard input ID
• A for attribute set via html: { for: "..." } takes precedence (||=)
• Works correctly with nested forms (simple_fields_for) — field_id sanitizes the full object_name path consistently with Rails' input ID generation