Refactor `si-accordion` to use `@angular/aria/accordion`

[chintankavathia]

Background

The accordion currently manages expanded state, trigger/panel ARIA relationships, keyboard
behavior, and IDs itself. The @angular/aria/accordion primitive can provide those interaction
semantics while Element continues to own its structure, styles, animations, services, and
public API. The evaluation branch contains a parallel accordion-with-aria POC.

Related to #1070.

Implementation reference:
angular/aria/evaluation

Scope

• Integrate Angular ARIA accordion group, trigger, panel, and content primitives into the
  si-accordion and collapsible-panel components.
• Remove manual ARIA relationships, IDs, and keyboard/toggle logic made obsolete by Angular
  ARIA.
• Preserve vertical, horizontal-collapse, and full-height behavior.