Issue #3605741: Use site select for the exposed redirect Site filter
The exposed Site filter on the redirect admin view used a raw text input requiring the site ID. Switch it to the site_select element so admins filter by site label. Keep the filter hidden on non-default sites, which are already scoped to their own redirects, and guard the view lookup against non-ViewExecutable values.
Adds Unit test coverage for the exposed form alteration.
Closes #3605741