Add settingsSummary() to the ui_styles_attributes source plugin

Problem/Motivation

The ui_styles_attributes source plugin (Drupal\ui_styles_ui_patterns\Plugin\UiPatterns\Source\AttributesStyles) does not implement settingsSummary(). Any UI listing sources by their summary shows nothing for it, even when styles are selected.

Related: #3568363 in UI Patterns, which aggregates each source's settingsSummary() into the component summary.

Steps to reproduce

• Enable ui_styles_ui_patterns.
• On a component prop of type attributes, pick the Styles source and select a few styles.
• The source has no summary, while other sources report theirs.

Proposed resolution

Implement settingsSummary() in AttributesStyles: inject plugin.manager.ui_styles, read the styles setting, and return the selected options labels.

Both config structures are handled: the legacy _ui_styles_extra shape via UiStylesUtility::extractSelectedStyles(), and the current selected key. Unknown style plugins are skipped; an unknown option falls back to its raw id.

Remaining tasks

• Review the MR.
• Decide whether the legacy _ui_styles_extra branch is worth keeping here, or dropped with the rest in UI Styles 2.

User interface changes

The Styles source now reports a summary of the selected style options.

API changes

None. AttributesStyles gains a constructor and create(); the plugin is not meant to be instantiated directly.

Data model changes

None.