feat(#3452812): add widget settings for token-based default alt/title values

Stuart Clarkrequested to merge
issue/imagefield_tokens-3452812:feature/3452812-default-value-ui into 3.2.x

Add default_alt and default_title widget settings with Token module browse link support. Replace the fragile $field_settings['default_image'] fallback in process() with widget-settings-based defaults.

  • Add defaultSettings(), settingsForm(), settingsSummary() to both widgets
  • Pass #default_alt/#default_title element properties from formElement()
  • Remove unused FieldConfig::loadByName() from process() methods
  • Add test coverage for default value behavior and stored value precedence

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