Have more specific aria-labels for columns

As there can be multiple columns with link provider dropdowns, it would be better to make the label more specific — for example:

:aria-label-combobox="t(
    'tables',
    'Choose link provider for column {column}',
    { column: column.title || t('tables', 'Unnamed column') }
)"

Originally posted by @silverkszlo in https://github.com/nextcloud/tables/pull/1977#discussion_r2276760090

With a generic label like “Choose link provider”, a screen reader user wouldn’t know which column they’re editing if there are multiple link columns. Including the column title makes each field uniquely identifiable and usable. I don’t have the full picture of how accessibility is usually handled in the app — it could be worth considering more broadly later — but in this case it feels especially relevant, since otherwise the table isn’t really accessible for screen reader users.

Locate the column link provider dropdown and the existing generic “Choose link provider” aria label. Update the label to include the column title, using “Unnamed column” when needed, then verify that each dropdown is uniquely identifiable to screen reader users.