Label empty font options as “Default” in Classic chart settings

[Copilot]

Font family/size dropdowns in the Classic chart settings currently expose a selectable empty option with no label. This updates those empty options to display “Default” while preserving the existing empty value behavior.

• Scope

  • Classic editor only (no Gutenberg changes).
  • Applies to both chart libraries where this UI appears.

• UI changes

  • Replaced blank placeholder options (<option></option>) with localized Default labels (<option value="">Default</option>) in:
    • Google charts general font family/size settings
    • ChartJS general font family/size settings
    • ChartJS legend labels font family/size settings
    • ChartJS linear axis font family/size settings (X/Y)

• Coverage update

  • Added a focused E2E assertion in tests/e2e/specs/admin.spec.js to verify fontName and fontSize first options render as Default in Classic settings.

echo '<select name="fontName" class="control-select">';
echo '<option value="">', esc_html__( 'Default', 'visualizer' ), '</option>';

Screenshot reference from issue report:
https://github.com/Codeinwp/visualizer/assets/86592992/34ea21e2-5e7c-487a-aade-9c08bad69047

[HardeepAsrani]

This was only for testing, so closing it.