Add i18n override coverage for single- and multi-select labels/ARIA text

[Copilot]

This PR adds targeted test coverage for the recently introduced i18n override props on select components, ensuring customizable user-facing and accessibility strings are actually reflected in rendered output.

• Scope: new i18n assertions in existing test suites

  • src/vscode-multi-select/vscode-multi-select.test.ts
  • src/vscode-single-select/vscode-single-select.test.ts
  • No source component logic changes; only test coverage additions in existing top-level describe blocks.

• vscode-multi-select: covered override behaviors

  • selected-text updates badge text for both non-zero and zero selected states.
  • open-button-aria-label updates combobox toggle aria-label.
  • no-options-text is rendered for empty filter results.
  • create-option-prefix updates creatable placeholder text.
  • select-all-title / deselect-all-title update tooltip titles for bulk action controls.
  • accept-button-text updates accept button label.

• vscode-single-select: covered override behaviors

  • no-options-text is rendered for empty filter results.
  • create-option-prefix updates creatable placeholder text.
  • open-button-aria-label updates combobox toggle aria-label.

Example of the new assertion pattern:

const el = await fixture<VscodeSingleSelect>(html`
  <vscode-single-select combobox open-button-aria-label="Öffnen">
    <vscode-option>Lorem</vscode-option>
  </vscode-single-select>
`);
const btn = el.shadowRoot?.querySelector<HTMLButtonElement>('.combobox-button');
expect(btn?.getAttribute('aria-label')).to.eq('Öffnen');

[wfabr]

Superseded by PR #1, which now includes the i18n override tests.