fix(aria/combobox): allow setting readonly

[ok7sai]

Fixes #33352

[github-actions]

Deployed dev-app for 213728a to: https://ng-dev-previews-comp--pr-angular-components-33364-dev-vufoprht.web.app

Note: As new commits are pushed to this pull request, this link is updated after the preview is rebuilt.

[rudzikdawid]

Thanks for restoring readonly! However, it looks like the disabled state is still not behaving correctly.

I tested the dev-app preview, and in the "Combobox with disabled" section, I was still able to click the input, focus it, and type text ("i can write here").

Current behavior: A disabled combobox accepts focus and allows text editing.

Expected behavior: A disabled element should be completely inactive — it should be uneditable and unfocusable (skipped during tab navigation). For comparison, in the standard Angular Material Input examples, the disabled control is correctly excluded from the tab order and cannot be interacted with.

[ok7sai]

Thanks for reviewing! This PR is an early draft that needs to be polished.