MudDialog: Wire title into accessible name via aria-labelledby

[Copilot]

Screen readers were announcing opened dialogs as generic “dialog” without the dialog title, making context unclear on open. This change gives MudDialog an explicit accessible name by linking the dialog container to its rendered title element.

• Accessibility wiring in dialog container

  • Added aria-labelledby to the role="dialog" element in MudDialogContainer.
  • Added a stable, per-dialog title id and linked it from aria-labelledby.
  • Kept behavior conditional: when header is hidden (NoHeader), aria-labelledby is omitted.

• Targeted dialog coverage

  • Added unit coverage to assert:
    • titled dialogs render aria-labelledby and that it resolves to .mud-dialog-title
    • headerless dialogs do not render aria-labelledby

<div role="dialog"
     aria-labelledby="@(GetHideHeader() ? null : $"_{Id:N}_title")">
    @if (!GetHideHeader())
    {
        <div id="@($"_{Id:N}_title")" class="@TitleClassname">
            ...
        </div>
    }
</div>